Don’t buy a used iMac. There are fan problems plaguing more than 1/3rd of the G5 iMacs, and the only fix for it can only be done during your Apple Care coverage, and is a gamble as to it’s success. The added bonus of performance in the Intel is another good reason.
I’d buy a used machine in many cases, but the money would really have to be right, and I’m a big geek and can handle my own stuff. I’d get a machine from the Apple store (refurb), you get all of the benefits of Apple’s new warranty, and the machines have already gone through a break/replace/qc process, so your odds aren’t bad at all.