I was thinking about upgrading my graphics cards from two HD 5870's to either two HD 7970's or two GTX 670's. Right now I have an 850W power supply, which probably isn't enough for those two cards. What PSU might you recommend for the cards I listed? I heard the GTX 670 and 680 are very close in performance and both are very energy-efficient. Is this true? OR I could just buy an HD 7990 or GTX 690.