I'll keep the iMac until it gives up the ghost completely, so resale value isn't a parameter. It's more a question of "will the $400 give it enough longer life to make more sense than buying a new one now".

Perhaps I should go with a 500GB SSD and transfer the movies and whatever else is taking up space to the server. And then buy a new iMac once they put the next-gen GPUs in them.