I'd would also hav e to recommend the HD 7950 over both of the other choices. The XFX HD for example http://www.newegg.com/Product/Product.aspx?Item=N82E16814150590
Is cheaper than both the GTX 580 and the GTX 560Ti SLI but when you look at the benches you come to realize that all three setups are really close in performance. The only deciding factors are power consumption, price and multi monitor gaming support. It's nice to be able to run 3 monitors off one card, a capability Nvidia's next Gen. cards sorely need.
The real question is what are you using now ? In a few more weeks Nvidia is supposed to release their next Gen. cards.