If you don't need the All In Wonder features (TV Tuner) then the 6800GT/GS is by far the better choice in my opinion, it has a much better overclock overhead (at least the 6800GS has) and even without overclocking, the GT/GS is faster in OpenGL games and slightly slower in some DirectX games not too mention the GT/GS has HDR and Shader Model 3.0 and who doesn't love a good dose of eyecandy now and then.
Of course, if you are in need of a TV Tuner or could put a TV Tuner to good use the X800XT is not a bad choice, but strictly gaming wise, I would pick the 6800GT/GS.