I Just changed my GTX260 with a GTX295, but instead of seeing an immporvment the result was just the opposite. Games that where running perfectly on GTX260 run very slow on my new card. Then I realized that even old games like Prototype run really slowly. I updated my BIOS and reinstalled the drivers.
Well, it could be a couple of things. The problem you run into when buying used cards is if the card is in fact bad, you're screwed. Even with a lifetime warranty (like EVGA KR cards) only the original purchaser can submit an RMA.