Would I be able to throw a second gtx660 in later without upgrading my power supply
If you get a quality unit, are overclocking at or near stock voltage, and don't have a crazy amount of fans or drives, it should be enough. TDP for 660s is 140W. TDP for 3570 is 77W. Max stock power usage is near TDP, so if you had SLI that would leave you about 240W for the rest of your system and overclocking.