I was wrong about Windows XP 64-bit drivers (with the X850XT)..the latest you can employ is Catalyst 5.4. Anything 5.5 or beyond doesn't install properly. It's a known bug and ATi is working on it..
Damnit.
The #1 reason why I'm going with ATi (e.g. X850XT) this round is because of the steady driver releases. For a primary Windows XP 64-bit user such as myself, it's essential to have consistant updates. I know it won't be difficult for me to give ATi feedback - I have a feeling it's not the same...