Well, as the name suggests..
So many people have done mods to make the B&W G3/G4s (And some have done it commendably well! :)) But I wanted to do a mod on a bit older machine.
So I just bought a PowerMac 8500 on eBay. The case is in shoddy condition, but well enough that it'll make the perfect specimen.

Previous Plan:
My idea:
Take a cheap Mac Mini (i.e. 1.42Ghz G4, 512MB RAM, 80GB, Combo Drive.. etc. Possibly a CD Mini if I can find one cheap enough)
Tear out the insides of the PowerMac
Extend the ports to the closest matching the vintage locations on the back of the machine.
Mount an ATX PSU to power the Mini
Wire the front panel to boot the Mini
Mount the Combo drive (Or another drive if necessary) in the same location as the Optical drive in the previous setup.
Make the machine as light as possible by removing unnecessary internal case parts, etc.
Improve Internal Cooling
Mount an external 500GB HD inside the PowerMac and have it powered by the ATX PSU.
Paint the newly refurbished machine to seal the deal.

As far as the Mini itself goes, I can make the PowerMac it's "house" and have somewhat of a dock for it set up, with a cradle and such. OR I can remove everything from the Mini case (And use the case for another mod! :)) and mount the internals in the PM case... I'm undecided.

New Plan:
Okay, so it turns out that the 8500 they sent me, was actually an 8600. Even sweeter deal considering I paid 50$ shipped. Two 5400RPM SCSI 4GB Drives, Plus a 9GB I had a around, CD-Drive (Woooooo. :() and about 400MB of RAM. It doesn't boot on it's own just yet, but with the help of my Mac OS 8 install CD, it's happy.
I ordered a 450Mhz Sonnet G3 upgrade. (Thanks eBay.. 20$!)
While I wait for that, I figured I'll have some fun with the Mach V 250Mhz 604e.
So the plan is.. to superclock the Mach V, with water cooling.
The Mach V was quite a powerful processor in it's time, and even stood against the low level G3s. With this machine having maximum RAM capacity of 1.0GB it's quite a perfect specimen for a basic webserver (Which I'll be needing quite soon) running YDL or SUSE.

Aside from all that, heres my basic reasoning behind this:
I'm getting a 450Mhz G3 to replace it. So if it dies, I lose nothing. (I guess maybe the 20$ I could've resold it for, which I seldom do anyway..My lab is spare parts galore.)
250Mhz has been proven to be a underclocked version of the 604e Mach V. Which means I'm safe overclocking to 350mhz with this card. The main problem I can see is voltage supplied to the chip is most likely lower on the 250mhz to save power and processor life/heat production. But with water cooling and improvements, as well as a bit of voltage probing..It could really go for the long haul. While I realize that 250Mhz could easily have a different chip/board structure, since it is after all the second lowest model.. But it won't stop me from trying. And like I said.. There is slim-to-none chance of losing something.

Will it be worth the work? Who knows. I love modding, especially breathing new life into old machines. My Color Classic Mystic and PowerBook 540c Server are proof of that. (Besides, What would an 11 year old machine be with water cooling? Extremely badass. Thats what.)

(Update 3/13/08: Machine is up and running Mac OS 9.1 off of one of the 4GB HDs. Test indicated that 4 RAM Sticks are bad as the machine will not boot with any of the 4 within it. I suppose thats why it was so cheap. Anyway, thats still 512MB RAM. Not too bad. And crazily enough, with 512 RAM Mac OS 8 says there isn't enough system memory to open Finder. Scary.)

(Update 3/15/08: Ordered 2x 512 (8 128MB Sticks) From eBay for under 25$ and snagged a 10.8GB Fast SCSI HD for 20$. I figure with a software RAID it should provide enough storage for usefulness. Looking into a 600W ATX PSU conversion. Probing the motherboard to see if it's possible.)

(Update 3/17/08: Just did some research and found out there is a 1.0Ghz G4 upgrade for this puppy. There may not be a need for water cooling overclock after all. And it would be cheaper, but I lose bragging rights. I'm torn here. Also, the 1.0Ghz will probably be crippled on the 50Mhz System Bus anyway. We'll see.)

(Update 3/28/08: Well, the G3 upgrade arrived in the mail. And while the speed difference to the 604e is noticeable, it's overclocking capabilities are very limited due to the design. So this may be mainly just a restoration project instead of an insane hardware mod. More research will yield results. Testing continues tonight.)

Basically, this project has depreciated in the priorities. I just started a new job, and really don't have very much time. The low priority of it, is going to turn it into a low-class Linux server. I'll post anything if it ends up changing, but it shouldn't for a long time. (7/21/08)

Almost every mod that included a mini people have left the mini in tacked and leaves it kinda lame. Yet your idea of making the power mac case to have a dock for the mini would be a neat idea. At the same time what would be the point? I would go ahead and take the mini apart and use the mini case for a future mod.

Yeah.. You're right about that. There would be no real point to having a Mini dock, it being a desktop and all. I'm going to CAD up some scenarios and see which one I can visualize being the best.

Heres a thought. Two mods in one kind of idea. Do a carputer setup and have the mini as a hotswap. Removable then able to bring inside and pop it into the case.

Just a thought.

Hmmm... I like the idea.
I've got a ton of 12V 7 and 5 inch LCDs so that would be perfect for a carputer.
And I already have a mount installed on my dash for them. Right now I've got the PS2 in there but the switch would be relatively easy. I will research the possibilities!

If you get stuck let me know. I got the carputer thing down like as if it's a part of the car.

How does the output look with the composite?
The LCDs are fairly low res.. (800x600) so I think my best bet there would be to boot directly into Front Row. Front Row should look good at 800x600.

Also, I was thinking of getting a 150W Sine Wave inverter for the Mini, or do they make DC-DC power supplies for them... Hmmm.. I'll continue researching.

I have a spot in my dash underneath my Stereo head that would be perfect for a Mini. I'll draw up some diagrams, take some measurements and ponder a bit on this.

My setup doesn't use composite and if it did even with 800x600 it would look very grainy. I use 800x600 VGA and when I had a mac in my car I used a program called AMP, it's like frontrow but has a ton more options and toys that come in handy for a carputer.

A power inverter will work but theres a DC-DC power supply for the mini for carputers. Just splice into the car stereo power or even go directly into the fusebox.

The only thing I'd be worried about with splicing it straight in, is the problem of constantly getting clean power to the Mini. Though it makes less sense to step up the voltage only to have the PSU step it back down again. Maybe a 12V power filter could do the job. I'm sure I could build one up with little effort.

I might ditch the Mini idea completely after thinking a while. I've got quite a few spare Macs lying around that can do some good getting put into action. Besides, such a powerful machine would do much better under my desk than in my car.

So this is the part of the thread where I take this in a completely different direction.
(I still have the 8500 coming, so I'll make something fun out of it. Or soup it up or something like that. But I'm changing the parameters.)

I have a PowerBook Pismo, fulling working and upgraded laying around with nothing to do. It wouldn't be too hard to build the Pismo into an LCD monitor case (or the like) with a built in USB IR receiver, and mount that in my car with a nice paint job. It would be a a lot cleaner, with a lot less cabling and power needed. Not to mention a lot less modding of my actual cars dashboard (Fun, but not in an 04', when I'm unsure of my abilities.)

Another problem... It seems that AMP's creator has ceased development and shut down his website. Still have 1.4? :-

That's a neat idea for the pismo, I did something similar with a old IBM thinkpad but my old car at the time couldn't handle it.

I have it somewhere, kinda odd that the AMP website is gone.