Jody has a ten-year-old iMac. He saw an ad for a 21.5" iMac for $375 refurbished. Leo says that it's always best to buy refurbished directly from Apple. It may cost a little more, but the peace of mind is better. This sounds like a big corporation dumped a bunch of older machines onto Amazon Renewed for sale. Sounds like a good price, though. It really comes down to what year it is, what OS version it's using, and if it can be updated. A Snow Leopard Mac, for instance, can't be updated to Lion. So it's likely a 2010 machine.
Leo also says for $1,000, he can buy a brand new one. So it's better to save up a few more months and get directly from Apple. And you could find one in the refurbished section of Apple too. Jody will want Quad Core at least. 32GB of RAM.