Quake III Arena is a real-world OpenGL benchmark that we have been using here at TweakTown for quite a while now because it has proven itself to be one of the best gaming benchmarks around to compare a wide range of different products. Quake III is getting very old, but is still one of the best ways of testing video and PC systems for any instabilities and best performance.
At 1024 X 768 both the cards are extremely similar, it's not until you move to 1600 X 1200 that you see a noticeable lead in the X800XT. With that said though at over 300FPS there is no way you could tell the difference in game no matter what settings you threw at the game.
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