I'm in the same boat with my PM6500: I have tried both a 3dfx Voodoo 2 (sold to me on EBAY as Mac-specific) and an ATI Rage 128 (from a B&W G3) but was not able to get either to work in 3D accelerated applications (e.g. Quake I, Tomb Raider).
With the Voodoo 2 I patched the internal graphics...