I think they will perform similarily... but I'd get the Nvidia. I'm a fan of Nvidia cards, plus since the 6800GT has shader model 3.0 it's more future proof and will have better image quality on games that use shader model 3.0.
uou could always get a 6800GS. they perform about the same as the 6800GT and cost about 50 less.
It really depends on what games you are going to run. While both are certainly fast for their class of cards, one is optimized to run better with DirectX and the other for OpenGL (don't recall which does what). As seen in benchmarks such as Doom3 nVidia will almost always beat out a similary clocked ATi card just due to that fact.
But if you are just going off of clock speeds and such, as others have already stated, the ATi X800 XT is faster than the 6800GT.
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