a. Getting a 2nd GTX460 will not add 'a little' more performance but 'a lot' more.
b. As already mentioned by a few members above, getting a superclocked version is not preferred. I would just get the exact same model. This will offer the best probability that both cards will attain similar overclocked speed, so wasting nothing or little away.
c. As you do not specify which Seasonic 650W PSU you have, I cannot check it for you. You just make sure it has four 6 (or 6+2) Pin PCIe connectors available as a GTX460 needs two of these.
If the PSU does not have enough natively, then you will need to use 'two-4 pin-Molex-to-one-6 pin-PCIe' adapter cables to make up for it.