The consensus at AnandTech.com is 6800 GS.
The problem I see with ATI X800XL (from Tom's Charts) is it doesn't support a number of new technologies. It has PixelShader 2.0 instead of 3.0, and it doesn't support HDR or SoftShadows. Other than the X800XL seems to perform slightly better in most cases (i.e. by about 5%).
And, the problem I see with the 6800XT is that it only has 8 Pixel Pipes , where the 6800 GS has 12. This can make a world of difference in terms of performance, and I think most guru's will say the more pixel pipes the better.
Here's an article by Guru 3d on this card:
http://www.guru3d.com/article/Videocards/286/1/
Their conclusion states that its definitely a good buy at around $150 at most retailers (MWave.com, etc.), but that the GS is even better for $50 more. So the decision is pretty much up to you and how much you want to spend on the card.