I'm running XP x64, but don't do 64bit unless you have more than 4 GB RAM :p, because 64bit applications use more RAM than their 32bit counterparts.

I'm running XP on my laptop too, but I could run Vista and strip it down to perform about just like XP does. Maybe I'll do that on my next laptop. And I'm sure they'll release a service pack 4 for XP as well.