Seasoned users of Virtualisation can skip this but there are a lot of people who assume you have to be running a 64 bit OS to use a 64 bit virtual machine -- this is NOT TRUE BTW.
So long as you have the Virtualisation feature enabled you CAN test 64 bit guest machines even when running Windows XP as a Host machine so there isn't any reason NOT to try Windows 7 X-64 even if you are currently running Windows XP.
Vmware server / vbox are free virtual machine software.
The only problem in running a Windows 7 x-64 virtual machine on XP is the fact that XP will only see a maximum of around 3.5 GB RAM so you won't be able to create a Virtual machine much bigger than 2GB.
However there isn't a direct relationship on the size of a VM and a Real machine -- 1GB Windows 7 X-64 machines run quite nicely for testing apps.
Incidentally with vmware workstation (version 7) - paid software - 3d and a lot of enhancements have been made to the virtual video drivers
so you can even run Windows 7 x-64 with full AERO as a VM on an XP host system.
(If you upgrade to a Windows 7 REAL machine your vmware workstation will still work of course).
So XP users - there's no problem if you want to TRY Windows 7 without all the hassle of dual booting.