power requirements are almost entirely dictated by your graphics configuration.
Peripherals like hard drives and fans take very little.
sli gtx760 will need more like a 700w psu.
A better upgrade would be to replace the GTX760 in the future with a good single card like a GTTX780.
Realize that your cpu is not that great as a high end gamer. You are likely to run out of cpu before you run out of graphics capability.