If you can find a good deal on a 6800 I'd suggest getting one, whether it be ultra, non-ultra or GT.
The 6800 gpu is a very strong competitor with the x800 but an x800 xtpe, xt, or pro would do you just fine, I'm sure. I heard from a friend who owned both whom I sort of trust more than most ppl's opinions that he didn't like his x800 pro as much as the 6800gt so if it comes down to it I'd suggest the gt over the pro and right now the gt out of all the cards has the best price to performance ratio so that only backs it up.