"Most Windows XP and Windows 2000 users have the 32-bit version of these operating systems. For example, Windows 2000, Windows XP Home, Windows Tablet Edition and Windows Media Center Edition only come in 32-bit versions. For these operating systems, there is no upgrade installation path available when you upgrade from a 32-bit operating system to a 64-bit operating system. However, you can perform a custom installation that uses an upgrade license."
My reading of that is you cannot do an upgrade in place which allows all your settings and programs to be saved in situ. You can do a "clean install" (of a type) which "replaces" the XP 32 bit with Vista 64bit but all your files etc will be gone.
The general view of many is that there is so little about at the moment in terms of 64bit drivers and so on that 64bit installs are potentially hazardous and should be avoided.