I made fun of Mac forums in an earlier post, but if you haven't checked out the MacRumors Forums, you should because they have a still-active PowerPC section. Here are a few threads you may find useful:
F.A.Q. for Powerpc Macs
Video on PowerPC - Part 1: Playback on G4/G5
OpenMark results on PowerPC Macs
That last one is a long thread benchmarking graphics cards. Stay out of the flame wars. They could get hairy in there.