Actually, it does matter - I have an 850 Watt PSU that can't handle OC'ing a single S3 (barely handles stock speeds). It loses hash compared to now that I run the 850 one one blade using two separate cables from the PSU (i.e. not a single cable from the PSU that has two 6-pin PCI connectors) and one 600 on the other blade.
Yep, a single 650 will likely not be able to handle OC'ing one S3. IMHO.
Was that 850W a single 12V rail PSU?
You only need to read a few pages back to discover that miners (including myself) have successfully been OC'ing S3s with mere 500W PSUs with only 2 PCIe power inputs.
Yep, an OC'd S3 will pull a hair over 30A on the 12V line. Your PSU has to be able to supply at least 30A, but as long as it can do that, you're good.
My wimpy 850 was single rail, rated at 58A. The voltage stability and level on cheaper power supplies can have a lot of variance and can even be just under 12V on a multimeter. Don't trust ratings alone. ;-)