Go for the X850XT!!! it's the baddest card around!!!
It OWNS and I mean Owns the 6800GT/ Ultra.
X850xt = Pretty good card - got mine for $450
6800GT = pretty good card also. But it has one feature the ATI card does not: PS 3.0.
If you're not going to be able to upgrade for a few years, go for 6800GT, If you are going to upgrade in the next year get the 6800GT.
Nvidia has the upper hand when it comes to next generation games.
As for me, i still love my ATI and i will think about Nvidia when a game comes out that REALLY runs better on Nvidia. Give me about 6 months and i'm building yet another monster of a computer...........