Sounds like my first high end file server back in 98. A old AMD 486 DX2 133MHz with a Biostar socket 4 motherboard with built-in IDE and 2 PCI slots. I tossed into a old SGI server case with 4 IDE hard drives and 7 SCSI hard drives. I got around 90GB in all of space and used Linux to run the system. I didn't have anything special for running the system, I used a FTP and SSH.

Your system sounds sweet as heck but why would you need all of that hard drive space? The largest drive I own is 250GB and i've only used 20GB out of it. What are you using to power all of those drives? I doubt a DA power supply can handle all of that with out stressing out.

You're quite right, I will need to get a beefed up PSU and mod it to run the DA board I suspect, but they are pretty cheap these days.Suggestions of wattage are welcome. Could I run 8 (or even 16) drives and a DA from 600W?

As for whats going on it, the answer is my music and video libraries. But it will also be used to backup systems on my home network. I might run portable home folders off it too. Not sure just yet. The Xserve will probably end up running a mailserver among other things, so that will archive to the RAID too.

I have sooo much media its not even funny. I estimate I am around the terabyte mark at the moment, but if I had two or three more I could be even less picky about what I add to it.
The current terabyte is spread across 3 machines in two buildings on 9 HDs and another RAID (at work).
I just want it all centralised and fault tolerant.

At the 16 drive point, I would split it into two banks and use dual 600W supplies.

16 drives?  I would say thats overkill unless you run a web server.

Personally, for this project I would look into how much money I had and look at how much storage I really needed.  I must admit that I am not familiar with what a "DA" is.  However, unless a DA has at least two 64-bit 2GHz processors, I would not use it for this project.  It sounds like it only has PCI-X slots.  While this is not necessarily a bad thing if you were to go with a new motherboard with multiple x4, x8, or x16 PCIe slots you would have the more bandwidth and less bus contention.  On Intel chipset server motherboards, all of the PCI-X slots are funneled through a single controller that is only has a x4 electrical PCIe width and external PCI-X Magma expansion chassis also only use a x4 PCIe slot.  In my research on SATA RAID setups, if you want to get say 4TB with RAID 0+1 for fault tolerance (RAID 5 at these prices would use the CPU for the XOR operation and would be very slow as a result).  So for this you would need sixteen 500GB drives or eight 1TB drives.  For the drives alone you are looking at about $100 for each drive, so roughly $1600 for the drives alone.  Then, you would need a case for these disks.  On newegg.com there is a pretty barebones case in the external enclosures section from AMS that has four drive bays and a four port SATA port multiplier which runs $200.  This is the least expensive case on Newegg.com with a SATA multiplier.  Buying four of those would set you back another $800. Add in the Norco SATA PCI-X card for $80 and you are looking at roughly $2500 or more if you want 8MB cache drives or 16MB cache drives.  If you think that four of those cases would be too big, you could use a PC tower case and take the SATA multipliers out of the cases and put them in the computer case with the drives.  It would require some hacking, but this is macmod.com.  If you wanted fewer drives I would use 1TB drives, however you are looking at roughly $3200 for a similar setup as above with eight 1TB drives at $350 each.  I think that if you wanted fewer drives or even if you have some time, that you should consider waiting until 1TB Samsung F1 series drives are available.  These 1TB drives use three platters which should be more reliable than the four or five platter 1TB drives from their competitors.  They supposedly were going to be shipped to OEM customers on 8/16/07, so they should be available within a month.  However I doubt that the Samsung drives will be less than $250 which is the price eight of them would need to be to be the same price as a similar sixteen drive setup using 500GB drives.  I'd also wait to see the Samsung F1 series drives because the 500GB drive might cost less than an equivalent drive from a competitor.  I am not trying to sound like a Samsung salesman, but I guess I like their drives because in the past Samsung drives have been generally less expensive and I have had no quality problems with them.  Also as the F1 series have a maximum platter density of 334GB, highest in the industry so the drives should be more reliable than their counterparts which use more platters and heads.  Also just to sound like a broken record, I'll mention it again as it seems to have been suggested as an option several times, RAID 5 at the low end is always going to have the XOR operation done by the CPU and will be slower that molasses in January, I don't think it should even be an option you even consider.

PCI-E didn't come out for a Mac until Apple switched over to the Intel CPU. The DA (Digital Audio) is a older G4 system. It uses a 133MHz FSB and uses PC133  SDRAM. Check this out. http://www.macgurus.com/productpages/sata/lycesata4.php

I figured that that DA referred to the "Digital Audio G4", but sorry, I just didn't expect that someone would even seriously think about using anything less than a 64 bit CPU like a G5 or an Intel Xeon system.  Besides most older systems probably cannot properly deal with filesystems above 2TB.  Even though Sonnet makes a dual 1.8GHz G4 upgrade for the DA, a Dual 1.8GHz G4 would be insufficient for any serious server.  The maximum I/O throughput of both the CPU bus and the throughput of the 66MHz 32-bit PCI interconnect of such a system would be too low for a reasonable server, especially with RAID 5.  Yes, the PCI slots are 33MHz/64-bit but the connection for the south bridge and many of the peripherals is 66MHz/32-bit PCI.  A PCI to PCI bridge chip is used to bridge 66MHz/32-bit PCI into 33MHz/64-bit PCI on the DA.  The reason that 66MHz/32-bit PCI slots never caught on with x86 computers and even other Macs is that the timing and signaling requirements are too high.  PCI is a load/store architecture like a RISC processor and the decode access window for instructions on 66MHz PCI is 2 ns.  This is not very much time and hard to design for, AGP and PCI-X slots add time for certain operations and relax signal requirements.  Besides, the Gigabit Ethernet on the DA is slower than a modern PCIe GigE card.  As much as I like Macs, literally for the cost of a 1.8 GHz DA upgrade from Sonnet, $500, one could for exactly the same price, buy, two generic 1GB DDR2 800 DIMMS, an Asus M2R32 socket AM2 motherboard which uses the AMD 580X chipset, a top of the line AMD Athlon 64 X2 6400+ dual core Socket AM2 processor, running at 3.2GHz.  Such a system would not even be somewhat comparable to a DA, the I/O throughput would be much higher and such a system would in general be much faster than a Dual 1.8GHz DA.  Granted, the AM2 system would still need a CPU cooler, case, PSU(s), video card, but still one should be able to pick those up for less than $240, the cost of four decent 512MB PC133 DIMMs for the DA.  Not that the DA could necessarily properly handle 2GB of RAM.

The case you mention probably would work fine, but I thought the AMS case would be better because I personally think that removeable drive trays on more expensive external cases are not of much use.  Besides the SATA multiplier chip used in every SATA multiplier on the market is the same and is from Silicon Image.  It also shouldn't matter which SATA multiplier case one buys, as there is no driver software necessary for SATA multipliers aside from the driver software used on the SATA controller.

Also since I am a nitpicker, there are PowerMac G5 PCIe systems, but they are probably not very common as IIRC they were not available for a long time and probably didn't sell too well as many people may have waiting to buy an Intel Mac Pro.

http://developer.apple.com/documentation/Hardware/Conceptual/PowerMac_G5_05Oct/index.html

A 64bit CPU setup is over kill just for a home RAID server. At the most a dual CPU setup would be fine but that is still a bit much. I've seen people use old IBM Pentium Pros as a home RAID file server and ran perfectly fine. It mostly has to do with the cashe on the drives and the RPMs. You can have the worlds fastest computer but with a 4800 RPM 4MB cashe drive it would run low as heck on file transfers and loading files into swap or VM. Even if the system was to use 10/100/1000 nics it wouldn't be much faster unless you had all of the systems connected to it with the same type of nics. In all it's the speed of the drives and the LAN traffic.

The Dual Core G5s had PCI-E. And, you definitely don't need a 64-bit system to run a large RAID or RAID 5 setup. I have a 3.5TB Xserve RAID running on a dual 500 G4 where I work. We also have a 7TB running on a G5. The G4 doesn't have that much of a performance decrease when your networked to it. It's all limited by the speed of your NIC anyway, as stated above. Sure, native things like a find copy to the RAID and such are slower, but thats only because the G4 doesn't have PCI-X.

The one requirement I didn't mention was that I wanted the system to be rack mounted. Thats why I chose this old UPS case. The drives will not be in modules, so not hot swappable. I originally thought of building a PC and running some flavour of Linux on it, but I do want reliable AFP from it. The DA parts have the advantage of being free, but will also do iTunes and iPhoto sharing.
I also agree that 64bit CPUs are overkill in a system where the PCI is going to be the bottleneck. The filesystem size is down to the OS (At least where Macs are concerned), except for hardware limits (DA will only do 128GB) on the drive controller chip, which will be on the RAID card in this system.
The system is going to hold 90% media, so it only has to be able to stream video with any kind of urgency. I'm not using it for video editing or as a scratch disk for CS. I know I mentioned fibre channel, but I doubt I would get much benefit from it over gigabit ethernet.
I don't know much about the chips in the Xserve RAID, but I doubt they are 64bit, and I doubt they are clocked very high, since these are dedicated chips, not general CPUs. They are probably just drive controllers with their own memory controllers and network interfaces added on. Which is why I reasoned a G4 at 700MHz or so would do a reasonable job. If I find myself accessing the RAID from two other Macs at once, I could add the dual CPU then, and should help with concurrent access, but I am unlikely to want to watch two videos at once.
As for choice of drive, I hear alot of people who swear by certain brands or models, but I see dead drives of all makes all the time. Such user testimonis may have been accurate when drives were at 20 or 30GB, but the bigger and faster they get, the less reliable they become. I always recommend buying the drive with the longest warranty, and having fault tolerance. But when you are talking terabytes on a tight budget, full mirrored backups or archives are inefficient and expensive.