if you had a G4 sawtooth (10 pin logicboard powerconnector), you could have used pretty much any ATX and adjust the pinout.
if you have any other G4, such as your quicksilver, you will need a PSU with one 28V lead. the board will not work without the 28V. this means: forget any ATX PSU.
in this case i might suggest an MDD-PSU. they give 380 or 400 W and are relatively easy to find on ebay due to apples past exchange program for noisy MDDs.
Pinout must be adjusted in any case!
btw: the old quicksiver PSU can be sold on ebay for a good price. there is a high demand as they seem to be the most failing component in quicksilvers....