The way XP and XP64 addresses memory is the same the main diff is that winXP is 32 bit and winXP64 is 64 bit...
This translates to winXP64 processes the memory in 64 bit lots...Therefore making it a quicker mchine...The catch is you will need 64 bit programs and there ain't a lot around, so therefore you will encounter a lot of incompatibilities...I know because I had it installed and experienced problems, so I switched back to the 32 bit version...
You will encounter problems coz Microsoft withdrew support for this operating system...They just extended Win XP for another 6 months after a lot of arm twisting...