One cannot "upgrade" a 32-bit O/S to a 64-bit O/S.
You would need to purchase a 64-bit version of Vista
and perform a complete clean install which requires reformatting
the hard drive.
You cannot do an in-place upgrade from XP x86 to Vista x64.
You start the computer with the Vista x64 dvd, enter the product key, use
your disk tools as needed to wipe the XP system volume (if that is desired),
and then perform a custom installation of Vista.