The X850XT would be far superior to the 6800gs. And especially since they're about the same price, you may as well cancel the 6800gs and pay a few bucks extra for a lot more performance. Especially since the X850 has 16 pipes compared to 12. Also in most of the THG benches X850 series pretty much beats the 6800s in about every bench (not counting HL2 and DOOM 3, of course, since ATI > NVIDIA as far as HL2 is concerned and NVIDIA > ATI as far as DOOM 3 is concerned). You can also think of the X850 as an investment that should last longer due to superior performance.
Good luck with everything.