Getback:
This advice pretty much goes hand-in-hand with what Bald already touched on: For the sake of full disclosure I'll start by saying that I've never used Windows XP 64 myself, but I have had several peers who went to the trouble of getting it. To a man, they've all complained about an ongoing, never-ending, litany of driver incompatibility issues. It seems many 3rd party vendors either do not, or will not provide direct XP-64 driver support for their products. Even Vista-64 is still grappling with this issue (though to a much lesser degree).
My suggestion would be that you first verify that there are XP-64 drivers for your hardware before you make any final decisions to take the plunge. Doing so does not guarantee they'll provide adequate performance, but at least you'll know whether they even exist before you dole out your greenbacks for a copy of the OS. If you have any "can't live without this" hardware, not having a driver for it would make your decision an easy one.