750W can handle 2 Tis, OCing or not.
A 560 Ti uses around 285W under load, so the two cards together will use a maximum of 570W. On a 750W PSU, that leaves 180W for the CPU (which uses 95W) and all the other parts, which don't use a lot of power.
My source: http://www.bit-tech.net/hardware/graphics/2011/01/25/nv...
Have you already bought the processor? A 2500K would be a much better choice. Overclocked, it will outperform a 2600. Free performance, and $80 cheaper!