Here are the Pinouts for most of the G4 PSUs:

http://www.outofspec.com/frankenmac/wire.shtml

Anything newer than a Blue & White is slightly trickier to convert from ATX, though the Yikes/Sawtooth is still fairly simple (Got this to work once). The GigE needs an extra 28V from somewhere which an ATX doesn't have. There are instructions on the site for this though.