Hey Speedmike.  Welcome to macmod!

The fastest PCI vid card for mac these days is the Radeon 9200 128MB.  Here are a few on ebay: http://search.ebay.com/search/search.dll...;amp;category0=

The radeon 7000 is almost as good though so it all depends on what you need.  The B&W's use a 66mhz PCI slot for graphics which is essentially AGP 1x so even if you could get a better card it would be wasted only being able to feed through a 66mhz bus.
Mac mini C2D 1.83GHz - 2GB RAM - 200GB 7200rpm HD
Sawtooth w/G4 1GHz - 1GB RAM - SIIG SATA w/2x1TB HD
B&W w/G4 600MHz - 1GB RAM - 2x80GB HD
Sawtooth G4 400MHz - 512MB RAM - 40GB HD
Dell Ultrasharp 2007WFP 20" Widescreen LCD
2.94TB Total Storage