Has anyone tried a newer 500 or higher watt DUAL SLI ready PC PSU to power an old G4 ?? I figure with the 2 SLI 12v lines this would be a easier attempt to get a almost TRUE MAC replacement PSU (yea i wont use ADC video, but heard powered firewire will suffer which i do want to use)

Is that an ATX2? As long as you can mod it so you have the right voltages in the right places and can make it fit, it should be fine.

I have put 585W ATX power supplies into a Sawtooth and a Digital Audio. With the DA I spliced in the front panel power board from an old B&W to enable the firewire ports. The new power supplies are much quieter, more powerful and less expensive than the OEM units. Wiring diagrams are out there on several different sites.

Out of curiosity why did replace the PSU with the ATX power supply not power the firewire ports?

My old B&W G3 is running 24/7 with ATX PSU (350W) for few years and I have no issue copying gigs of files over a 2.5" external drive powered by firewire.

I have also done it on Sawtooth. 400W ATX PSU. The only thing you need to watch is when you add faster proc and power hungry graphic card (for example ATi Radeon 9800).

The firewire ports work fine after startup but they are not bootable unless they are fed by the +28Vsb line that is always hot. ATX PSUs don't have a +28V line, their +5Vsb line isn't enough to boot firewire drives. The front panel on the B&W steps up 12V to 28V for the Smurf's firewire ports (on G4s this function was taken over by the Mac PSU). So a B&W front power panel can be spliced into an ATX PSU to make the equivalent of a Mac PSU. It steps the +5Vsb up to +14Vsb which is enough to boot from firewire drives (still not enough for ADC monitors, you will need to use a PC monitor). I found the directions here:
http://forums.macrumors.com/showthread.php?t=261623

That is when you add a power regulator and increase a 12 volt lead to 28volts.

My B&W has a 400watt ATX and my sawtooth had a 450watt ATX.
I even used the old B&W power supply for my headless iMac. Only system I couldn't get running with a ATX power supply was a digital audio.

The link I posted above is for a DA with the B&W front panel functioning as the power regulator. I set it up on a DA mobo with a dual 533 MHz processor, 1.5 GB RAM, GeForce 4 MX, Acard controller with ATA RAID 0 drives.