There is numerous threads on this. Do a search of TF and will find much more info. I will link you to some good threads that will help you make a good decision. But to sum it up, get the 6800 GT is a much better card than the X800XT, mostly because it features SM3.0, which will be required to play future games in DX9 mode.
Basically, game developers are now only developing games to use Shader Model 3 and are not including any other DX9 paths, such as DX9.0b. Because of this, if you have an X800XT, games will default to DirectX 8.1 compatability mode, where you lose both major image quality and a hefty amount of performance.
