My 3-year old iMac 800 is still (according to EveryMac) worth $1000US. I paid $1800 for it new. Somehow, with a $499US Mac Mini waiting in the wings, that seems a bit crazy.
Of course the iMac has a 15" flat panel monitor, Apple Pro Speakers, and keyboard and mouse. And it's upgraded with an extra 512MB of RAM.
Still, I'm thinking of selling/trading in the iMac rather than passing it on to the parentals. It might end up being cheaper that way.