Back when the 560Ti released, they were great value since you could buy two of them cheaper than a GTX580 and get equivalent or better performance. So I would start by looking at what the 580 can handle.
Agree with Mack though. There's not much point buying two old mid-range cards to SLI with. You can spend your money better on just a single modern card, as currently there arent any magic cards like the 560Ti was when it released.