The old one:
I'd use that as a file server or something! My iMac 500 does that. Anyways, congrats on the new Mac!
Or you could sell it to me.
Seriously though, I'd love that to use as a backup. But I already have my backup PC and my iBook.(But should those two plus my Mini ever fail all at once, I still have my iMac 233
But maybe you should use it for a backup, you know, just in case something goes wrong with your new iMac.