Get yourself a Mirror Drive Door Powermac. It was the last model before the G5 came out. They all will have at least a Radeon 9000 64MB in the AGP port and you can put a better card in with very little money. You can still get DDR modules and max your RAM to 2GB, its got standard PCI slots and the "airport" wifi slot has enough room to fit a standard sized PCMCIA card that compatible if you want wifi with it.
I paid $60 for mine and picked it up local. Also check craigs list and direct them to ebay if they are dreaming of getting a $200 or more sale from them.
Also you can put Leopard on them. Slow Leopard you can not however install.
