A GTX 660 will require require 140W and draw about 11.7A from the +12V rail at max load. Your PSU has a +12V single rail capable of providing 45A. Plenty. But 2 of those cards in SLI will be pushing it. A good quality 750W (or better) would be preferred. Having said that, I would not be surprised if the 600W CM PSU handled the 2 cards fine.