How capable are the OpenGL drivers under the DRI project[1]? i.e. How
does DRI on XFree86 compare to say the nVidia drivers, Xi graphics drivers
or Windows drivers of similar strength video cards?

I'm considering a Radeon 8500 or similar card, and if it can provide
adequate OpenGL frame rates, I'd be happy. Some simple comparisons with
the gears sample program would be interesting - at 1280x1024 with
unaccelerated Mesa, I get about 10 frames per second. The nVidia drivers
get much better rates, but that card died after the fan failed.

1. http://dri.sourceforge.net/