How much watts of power would I need to power two GIGABYTE windforce gtx 670 at approx 1.2 Ghz overclock with core i5-3570K?
NVidia Website itself put the power requirement of the 670 at 170W max per card. With a recommended system power requirement of 500W for single card configuration, I would say 700 as minimum for 2x670. Add 50W for overclocking to be on the safer side.
But there is also a PSU calculator that allows you to input your desired system configuration to estimate its power requirement.http://extreme.outervision.com/PSUEngine
It still does not have 670 in its drop down options for graphics cards but you can just enter 680 to be on the safe side. But I use this only to check if existing PSU can support the desired upgrade.