XP 64-bit had some problems. I don't know if SP3 address this. XP Cannot access more than 3.5GB of RAM (32-bit)
The best idea for 64-bit is Vista.
Most of the problems are driver problems on both Vista/XP x64. As long as your printer,etc have x64 drivers you should be fine. I run XP x64 on all my CAD/editing based rigs and it works fine. Also SP3 is not available for XP x64 yet.