Sniper,
The 6800 GT looks to be a very good card for the money (it's almost as fast as the 6800 Ultra). If you like nVidia, then definitely go for the 6800 GT. The 6800 GT outperforms the X800 Pro (for the same cost) fairly conclusively (however, the X800 Pro only has 12 pipes enabled, so the 16 pipe 6800 GT should be faster).
Otherwise, you could get an X800 Pro VIVO and update its Bios to unlock all 16 pipes and set the speeds to 475/450, 500/500, or 520/560 (although not all VIVOs can handle XT PE speeds). This is very easy to do if you follow w1zzard's article on this forum. You may also need to perfom a voltage mod to run at the XT PE's core speed of 520. This is also very easy to do. Some people feel ATI has better image quality, but this is rather subjective. I've not yet seen a comparison between the 6800 GT and a 16 pipe X800 Pro VIVO, but I'd guess they would perform equally. ATI does release new drivers quite frequently, which is always nice.
One final thought. It seems that the 6800 GT performs better (relatively speaking) on mid range cpus. To get the most from an X800 XT PE, you really need a very fast cpu (P4 3.2 GHz or higher, or better yet, an Athlon 64). This may be important depending on your system (I'm upgrading my P4 2.6 GHz cpu to a P4 3.4 GHz). The good thing is you really can't go wrong no matter which way you go. Good luck!
Edit: spelling