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If you want the most Frames Per Second, then the 2 560's in SLI would be better than a single 660, but there are other things to consider. Power consumption, heat, price, and driver support are all...

Rumors suggest a mid-August release.
You cannot run it in SLI with a 560Ti.

I would wait a little bit. Nvidia is scheduled to launch the GTX660 Ti very soon and the early rumors are that it will be a solid performer.