Malmental is right in many of his comments.
I agree that the 560ti SLi is a great performer as I'm running that myself , they overclock like mad and easily surpass the 580 and 7950 in some games....but if you are willing to spend more then I say consider getting a 7950, they are a great card and overclock very well .....but like malmental also said It would be wise to wait and see what Nvidia's 6xx series is going to be like especially if they sugest that they will be faster than the new ATI cards.
This is not a fanboy comment, I like both Nvidia and ATI, merely just stating that its wise to see what Nvidia does when they release the new cards , it will create some much needed competition to the market to force prices to drop from both Nvidia and ATI with new and current series cards....don't rush when its just around the corner.