I don't trust Xbench on this one at all.
The Mac is a Dual 1.25 GHz (FW 800) G4.
I have had 3 different grfx cards in this Mac:
1) Radeon 9000 (OEM)
2) GeForce Ti (was the best at that time, as the Radeon 9700 wasn't retail)
3) Radeon 9800 Retail
Xbench version 1.0 showed better OpenGL results with the Radeon 9000 stlil in there, running 10.2.3.
Just do a search on 'Ernout"... you can see for yourself.
I play games like MOHAA/SpearHead, F1 CS 2000, Nascar 2003, RTCW, Tiger Woods...
As you would expect, the games FPS improved using the better cards. Results did vary enormously, though. And MOHAA/SpearHead had bad visual artifacts with the nVidia. Bad drivers????
The Radeon 9800 is by far the best gaming grfx card for your Mac. Maybe the Xbench results weren't too favourable, but I can tell you the gaming experince WILL improve. I dont know if a sub-1GHz Mac will do this card justice, though.
Nascar (all settings maximum possible, resolution: 1600 x 1200) let my CPUs be the bottleneck (or FSB...).
What's especially fun, is the ATi control panel, that lets you override the FSAA and AF settings of the games you play (see pic) ! I play RTCW, with the game settings 1024 x 768, all settings maximum possible. And the ATi Radeon 9800 control panel, set the FSAA on 4 x, and AF 4 x.
It is awsome!