Recent Review @ digit-life
Chances are that for the most part, the gaming experience will be equal in most games. The thing is 6800Gt will be much faster in Doom 3.
For the $$$ it is not worth it to spend extra to get X800Xt, especially since 6800GTs can be found for much less than $390.
I would conclude that X800xt is faster, but we aren't talking about noticeable performance improvements.
If you do intend to keep this setup for a while, and you are replacing the whole rig, it might be worth it to wait for the Nforce 4 SLI setup. Even without it, in the next 2-3 years, chances are you'll encounter quite a few games using Doom 3 engine and that is when 6800 will be faster. As far as Direct3D games, ATI will be faster pretty much most of the time, but the difference will be much less than with doom3-engine-based games. You probably also know that in any game where there are a lot of complex shadows (ie Doom 3) and so on, Nvidia will be faster too.
I dont think lack PS3.0 is that much of an issue, except for HDR lighting if that interests you. Maybe ATI will incorporate 3Dc soon?
Either way, in 2 years both cards will run the latest games at 20 vs. 30 frames just like 9800Pro does at Far Cry and Doom 3 right now. So if I was you, i'd actually try to save $$$$ and buy something mid-high end now and upgrade in the next 2 years and so on...instead of putting down $500+ towards a card now that is maybe 25% faster than the next $350 offering but both of which will be inadequate in 2 years. If you put $150 towards the next upgrade next time you buy a $350 card, it'll only cost you $200 extra.