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Unfortunately you can not run a GTX 570 and GTS 250 together in SLI.
Fortunately, you do not need to!
With both cards installed (with no SLI bridges) you can select in the nVidia control panel to have the GTS 250 dedicated to PhysX.
While the Corsair GS700 should have enough power for your system and both GPUs, it is unfortunately one PCIe power connector short.
Where you will need a total of 3x 6pin PCIe power connectors (2x for the GTX 570 and 1x for the GTS 250) the GS700 only has 2x PCIe power connectors.