You can use an ATX power supply with some modifications. If you've bought a G4 motherboard, you can use your run of the mill PC-100 (or PC133 if it's a newer one) RAM.

And for a graphics card you can get an ATI RADEON 9000 Pro very soon for $150 or less. Dual-output no less. (Dual digital in fact - both ports can be used with analog adapter, one is provided). Or you can try your luck with a PC 8500 and then flashing it with the Mac BIOS. I don't suggest it though. Won't work for all 8500 configurations, due to memory differences (timings, not amount). Not to mention most PC boards have a 64KB flash chip and our Mac BIOS fits nicely into 128KB.

Have you thought what you'll do about a case? I may be able to get a spare one. Maybe.

Bruno