Good 650 w power supplies are 650w of CONTINUOUS power, which means they can handle load spikes up to 900w's without any issues. Also, most of these power supplies are 80% certified, which means that EVEN at a full 650w, they run at full efficiency.
If you don't use terrible quality psu's, you too can run oc'ed sli 460's on 650w.
None of those show total system power out put being above 500 watts. You would only want a more powerful PSU (more than 650W) if you were planning on stressing/torturing your CPU AND GPU together quite often.