I would not mess with the 64bit edition of Windows right now. It is a bit of a bandaid, and MS has all but said the 64bit version will not roll out until Longhorn ships next year.
There really is not much of a performance gain to be had.
Xtreme, I am not sure what to tell you about your system. You have a faster CPU than mine, same amount of ram, and same video card, and look at the score differences.
I think the only significant difference will be the way the OS is setup, but that would not account for that large of a difference between our systems.
Waffle, the X800XT has the 4 extra pipes, and slightly faster ram than the Pro. Right now, you are going to be CPU handicapped in most situations even with the Pro card. Your call.