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.