nVidia itself recommend 500W for a single 670.http://www.nvidia.in/object/geforc
For testing conducted for SLI 670, the resulting system consumption for SLI is 433W of which 293W are attributed to the gpu cards.http://www.guru3d.com/article/gefo
If you use this psu calculator considering your components, the result is 664W (overclocked processor & used 680x2 to simulate gpu OC:http://extreme.outervision.com/PSUEngine
All points out to 750W being more than enough.
Thanks to "bigcyco1" for doing all the work