provided your machine can run a 64 bit Guest virtual machine you can actually run very successfully a Windows 7 X-64 bit Virtual machine on Windows XP.
The drawback is that since the XP machine will only see 4GB of RAM the maximum size of RAM you can allocate for your Windows 7 virtual machine will be somewhere around 2.90GB.
However for testing various builds of Windows 7 with different AV software etc this is an OK testing frame if you are still using XP and don't want to dual boot.
It's good also for testing a lot of USB devices as well -- sometimes the usb device doesn't actually work on a Windows 7 HOST machine but since the USB interfaces are pretty well complete on an XP host you can see how the device works on Windows 7 as in this case the usb driver
will be fine.
So don't throw away your XP machine just yet - especially if your machine has 4GB or less of RAM. You can still do a lot of testing on Windows 7 x-64 BEFORE committing yourself.