850 watt for sure. Because the 670 is a power house!
The 670 takes less power than the previous generation. I have 2 560 448s and an overclocked 3930k, so my system will draw more power than yours and it never takes more than 500w out of the wall (which means 450w from the psu). Your system will probably draw less than 400. I'd suggest a high quality 650w from the recommended psu list in the stickys.
The power calculation sites always overstate power requirements, at least partly because many psus overstate their power output.
See this link for actual results: http://www.guru3d.com/article/geforce-gtx-670-2-and-3way-sli-review/4