Hi
Please see the following websites for the calculation
http://www.extreme.outervision.com/psucalculatorlite.jsp
http://www.coolermaster.outervision.com/
Also a system equipped with Intel Core i7-3960X, ASUS P9X79 Deluxe, 16GB Corsair 1866MHz etc etc, and SLI 760, consume at max 520 watt in gaming mode..
I recommend corsair gold rated 750 watt PSU. Good enough to run 2 way SLI.
Here is more
GPU STRESS TEST IDLE WATT PEAK WATT DIFFERENCE WATT IDLE WATT ESTMATED TDP
GeForce GTX 760 121 280 159 10 169
GeForce GTX 760 SLI 126 449 323 20 343
Let me know if this resolves your query.