Just adding a +1 to the opinions above.... Seasonic makes some of the very best quality PSU's out there, and the model you are looking at is a step up from their M12II 620w that I have used to easily power a pair of HD 6850's with an overclocked Phenom X4 965 processor.
There are 650w rated PSU's, and then there are PSU's that are designed to run flawlessly to at least their 650w claim. You have no worries.
I have the seasonic x660. It's perfect. PLUS the fan doesn't even turn on for the PSU unless i am gaming. It has a smart fan that monitors temp and load and only turns on when a certain value is exceeded (don't know the exact specifics)
You could even run sli gtx 570's with it, maybe even gtx 580's, so your lower ones will be no problem.