Hi there
remember that so long as the Virtualisation feature is enabled in your BIOS you CAN run a 64 bit version of Windows 7 as a guest virtual machine on Win XP.
The only drawback here is that the HOST machine can't see more than 4GB RAM (it's a 32 bit HOST OS) so your guest OS will have to have a smaller amount of RAM.
Most people forget this but there is NO PROBLEM in testing 64 bit OS'es on a Host 32 bit system so long as the Virtualisation feature is enabled in the BIOS -- this is pretty standard now in most modern computers.
Cheers
jimbo