I'd like to confirm what alk said...it is possible, no doubt, but its also very difficult, as the 8600 is based on a Tsunami form factor, whereas the G3 is based on a PC ATX form factor. It is possible most likely, but it would require SERIOUS modding.
Maccess: You most certainly can use any standard PC ATX power supply on a G3 very easily...the power supply just drops in, you don't have to cut any wires or mod it or anything, and there is a jumper on the mobo that has two positions: "MAC" and "PS2". For operation on an ATX PSU, move it to "PS2".

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