X850XT in terms of actual performance would out perform the 6800Ultra......regarding the whole Pixel Shader thing....god knows how long it'll be until it's actually desperately needed to enjoy a video game.....I don't see it happening any time real soon since lost coast isn't going to fully use it nor will a game like F.E.A.R.
Go for a GeForce 7800GTX, out runs the X850 Crossfire and 6800 Ultra SLI anyday. and also, with the 7800GTX, AA at 1024x768 is pretty much free, in some games there is actually a performance increase when AA is used.
The 6800 series is far from oldschool even though its been replaced by the 7800 series. The 6800 GT/Ultra still kicks arse in any game and has plenty of power left over. As the price falls on the 6800 GT, it will become the best bang for the buck. I'm even tempted to snatch one up, for it should get me another year or two when I will build completely new.