From reading a few articles it seems that using a PC ATX power supply will not work in a powermac that has a pro speaker jack.
This includes your computer..
The reason why is there's an additional bit of power the ADC port uses. Whether or not that would be the only thing you'd loose on the computer if you swapped it out with a standard ATX power supply I have no idea.
However the pinout is still different.. and it seems to change with models, so it might not be an option at all. you might want to look for a quicksilver or digital audio G4 power supply on ebay. Best of luck.