I too am now in the arena for changing out my power supply with an ATX mod.
I found a website with various atx to Mac pinout diagrams for various g4 models...
There is a way to spare yourself endless cutting and soldering of harnesses; use a 20 to 24 pin ATX psu cable adapter. two pin slots on MY model of mac will overlap into open space on the motherboard but the harness fits nicely this way with almost all the color coded wires being in the right spot.
From elsewhere on the net, cut a paperclip into two pieces, one end of each piece hammer-pounded flat and thinned on the wide edges to wedge down on either side of the pins inside the harness. Wires will then slide back out of the harness easily once you get the hang of the first one, means no increased resistance of soldered cut wires, and saves tons of time. Just redirect the colors into the right positions. Neater cleaner way of doing it.
The only problem with MY particular model of Mac pinout, is that the author suggests a pair of 3a diodes be used for wiring up a +5v trickle (to maintain memory during sleep) into a 12V line that provides operating power. Makes more sense to me that he MEANT for the atx 5v trickle to patch into the mac 12v pin position, but his diode diagram says into the g4 TRICKLE POWER position (only thing on the mac side with the word TRICKLE in it is the 28v trickle pinin... 5 and 12 into a 28v pin position? and this is ok in what way?)
He gives pinout numbers for all the rewiring of a psu harness, but neglects to outright SAY to go ATX pinout 9 (5v) to Mac pinin 22.
I do not have a cinema display so this line isnt really necessary as far as I can tell, however if the motherboard is expecting a steady 28v but only getting an alternating 5 and 12 volt signal, isnt that just illogical? I have not gotten word back from the guy on this lack of clarity. He also suggests the POK pinout 8 on ATX PSU go to a 3.3v in on the Mac side... which makes no sense to me. There is a 3.3v for every one of the mac 3.3 volts required with a spare leftover...
Finally, if this diode solution is to provide deepsleep needed for use with an ATX PSU, does that mean Apple made manufacturers build custom PSU's for their Mac Pros that had diodes/alternaiting voltage lines built right into the PSU?
If all this does is maintain memory in the mac during sleep phase, I am about ready to forget that nonsense and just tell my folks to shutdown every time they are done and forget about 'sleep' mode ever being available again.