You mentioned above that you don't have a PC with AGP, I was in the same scenario and just picked up an old dell PC off ebay for £6 and used that for flashing.
Get a Radeon 7500. I have one in my Cube, it works great. It is also ADC, if taken from a Quicksilver G4 as mine was.
The only caveat is no backplate unless you mod it. No VRM move needed. It works decent in Leopard as well as OS 9.