|You really don't want to do that.|
One problem (among others) is driver availability. 64 bit XP was never very popular and availability of drivers is very spotty. A 64 bit OS must use 64 bit drivers - no exceptions.
A much better option, assuming your hardware is compatible, would be to install 64 bit Windows 7.
Note that there is no upgrade path from a 32 bit to 64 bit OS so a clean install of the OS and all applications would be necessary.
But do you have a compelling reason to go to 64 bit?