I dual boot XP pro (32-bit) and Vista (64-bit) on all my machines.
It's actually pretty easy to do, the only big thing to keep in mind is that when  installing Windows XP on top of an existing Vista install, it will screw up Vista's bootloader, preventing Vista from loading (it will just go straight to XP as XP's bootloader cannot see Vista).

There are a couple of free utilities that will restore Vista's bootloader (I use EasyBCD - just do a google search for it) and set up the dual boot options properly.