I picked up a used EVGA FTW 670 right here on the forums for $320 and registered for a 3 year warranty on their website, they also me registered my EVGA 2WIN GTX 460 I picked up on ebay and I got the benefit of their RMA no questions asked.
I've seen them for $300-330 used, and if I had my preference I would take an EVGA because they've been so open about 2nd hand users and warranties. IMHO
Dry