I run XP, XP x64, and Vista x64. My wife runs XP and Vista.

I've noticed no difference between Vista and Vista x64, but XP x64 is definitely more troublesome than XP - though this has gotten better since the advent of Vista x64.

At any rate, I would not recommend XP x64.

Finally, for a notebook, I'd think carefully before installing a non-OEM OS on it. Make sure all drivers you'll need are available for the OS you select.