If you have to pay retail or near-retail price for x64, I'd say don't waste your money at all. Luckily I was able to get it extremely cheaply through my university, but even then I hardly use it...it's useless to me as a day-to-day OS since drivers for my HDTV tuner do not exist for x64 (and possibly may never). Even if you have hardware that is well supported, I don't think it's worth the money - you might notice minor performance gains in some situations, but nothing earth shattering unless you need to use tons of RAM.
You could always play with the trial as mentioned above and make your decision then, but as I said - if you're going to have to pay a lot for it (I have no idea what price x64 goes for at retail), don't bother.
Oh, and if you do play with the trial, don't install it to the same partition as your regular XP install, even if you use a different folder...I learned that one the hard way
