The one thing that I'd be careful of though with the two hard drive solution though is XP's checking of hardware. What your suggesting would probably work without a hitch (or few hitches) under Windows 2000, but under XP, XP's activation is tied to the hardware in the machine. So if you pull the drive and drop it into another machine, you may have to reactivate XP.
Once reactivated though, it'd be fine..