Hello,
What are you going to be doing with the PC? We can not give you suggestions cause we have no info at all.
Do you play on gaming? Do you do movie conversions? Are you doing anythign with Audio?
XP X64 has very poor driver support so your first task would be to make sure you have 64 Bit drivers available. Your next task will be to see if Vista Ultimate is right for you when Vista Home Premium is a good choice as well and costs much less while still being able to do much of what Ultimate can aside from the Extras which are not worth the money.
Cheers,
Mak