okay, jebus... first of all x800 is 12 pipelines, and 6800gt is 16 pipelines. the x800 xt is notorious for it's image quality and apsects of such. the gt6800 is not so much made for image but for performance..
therefore more fps will be pumped out with a shittier visual.. by shittier comparing those two cards.. i mean it wont matter.
lol honestly man, both are great cards. the x800xt is not designed entirely for games, it's original sole purpose is for game designers and such for private home use. thast why ati has those crazy shader 2.0 blah blah blah crap. so visuals can be better when creating games
the nvidia cards are designed so you pump out more fps and get a good gameplay.. so it runs smoother and more compatibly with games.
i'm an ati fanboy.. but i'm not an idiot. go with the 6800gt. it will rock ur gaming world.