I have a couple of Mac Radeon PCI models (original and 7000) in B&W G3's where they run at 66mhz (same speed as 2x AGP), they do ok. The Rage 128's that were in the B&W just got moved down to the PCI powermacs.

You do need to figure out if you will be gaming or just want snapier video, and what OS you will be using from 7.5.5 to 9.2 or OSX (not recomended).