It depends on the GTX 970; I used the MSI GTX 970 4GB that could draw over 200 watts at peak. Presume that a GTX 970 can draw from 170 to 220 watts and add whatever else is connected to the 12 volt rails. Since the GPUs could draw up to 35 amps (not very likely, but possible when running stress tests), you need a PSU that provides at least 45-50 amps on the 12 volt rail. Presuming you have this PSU: http://xfxforce.com/en-us/products/ts-gold-series-full-wired/ts-gold-series-750w-psu-p1-750g-ts3x it provides 62 watts on the 12 volt rail and it never will operate at full capacity. The same 650W PSU (53A on the 12V rail) would be enough, but you should get the 750W if the cost difference is minimal.