DO NOT get the X850XT. Get the 6800 GT thats much cheaper and outperforms it, plus has Pixel Shader 3.0. I can personally tell you that this generation of ATI cards suck, I bought an X800 Pro and have had trouble with it from day 1. Also, there is a guy on this forum who bought an X850XT (dhw200), and he is unhappy with it as well.
Pixel Shader 3 is what PC graphics is all about these days, even ATI's own HL2 is going to require Pixel Shader 3 in the near future. If you get an X850XT, you will be kicking yourself in the head when you realize how much you've been ripped off, and you load games to see settings such as "HDR, Hi-res water, Parallax mapping, Specular lighting," and more all blackened out.
The only way to go these days is the GeForce 6800 Series. Don't walk from ATI, RUN FROM THEM!