I just went through a similar process. I upgraded my main PC and put its motherboard, CPU, and RAM into my backup PC.
The backup had XP with all service packs on it, but I had an unused retail Vista license and decided to put Vista on the backup. The only real reason I bothered to do that was because XP is no longer supported by Microsoft, but Vista will still be supported over the intermediate term. Since I changed motherboards, I knew I would have to reinstall anyway, and therefore figured I may as well move it up to Vista.
You have to consider the circumstances under which you might use the backup machine.
Only if the main PC is broken down and even then only for a few days till you can repair it?
Periodically, even if the main PC is working fine?
And you have to have some plan to get your necessary data on the backup. When I rebuilt the backup PC, I installed antivirus, a browser, email program, Office, all available Vista and Office updates, and my most important data as of the rebuild date (Jan 29). I ran a checkdisk on the drives and used the backup for a few hours to confirm it was running well and configured properly So, I should be able to start it up and do something productive within minutes of any failure on my main PC.
A primary reason I keep a backup around is that Newegg takes ONLY online orders. If my main PC has a major hardware failure, I need a way to quickly order replacement parts--through the backup PC.