I hate it when my back up machine goes down. It never happens at a good time!!!
I would consider buying a great machine from Russ here on the board, perhaps even slightly better specs than the one you currently have, and make you're older one your new back up. Give it a major over haul and learn a few things in the process, then it should be ready as a back up, or any sudden expansion plans that might come your way!
That is, if cash permits!