Dont get either. The X800 Pro flat out sucks donkey d***, i know, I used to have that card. It gave me so many headaches, and it never played a game correctly because of driver problems. The X850 Pro is only a bit better because it can be flashed to X850XT. But really, you would be way better off going with a 6800GS. It wipes the floor with the X800 Pro and beats the X850 Pro as well.
Please, don't make the same mistake I did.