Well all though the core speed of the 6800gt is lower than the x800xl they are pretty much on par with each other. They get roughly the same 3d mark 05 score. The only significant difference is with hl2 and doom3 (where the x800xl is king with hl2 and the 6800gt is king with doom3). The only major difference between the two is that the 6800 has sm3 whilst the x800 doesnt, but the x800 is also cheaper. Heres a comparison chart that has both the graphics cards in it (link). It shows how both perform. Hope this helps.