Specifically, you cannot upgrade in place. A 32-bit OS installation cannot be upgraded to a 64-bit installation. You would have to save all your files somewhere and do a fresh install of 64-bit XP. Then re-install all of the software that you had on your system, some of which may not run on a 64-bit system. As Hawkeye22 stated, drivers for this OS version are thin.
Also, your hardware would have to support 64-bit software. That would depend on what kind of machine you are starting with.
Any particular reason that you don't want to start over with, say, Windows 7 64-bit, which is stable and has good, if not perfect, driver support?
So tell us. What kind of hardware (make and model, or CPU and motherboard if you built it) do you have? Do you intend to do a clean build, or did you want to upgrade it while keeping all of your files and programs (not possible)? Why do you need 64 bits; maybe someone could suggest an easier alternative.