So if you have 2 S3's and they're overclocked and pulling 374w each, then that's about 750w, so you should be looking for something in the range of 1200-1500w in platinum or gold rated.
A lot of people from the various hardware overclockers forums like to keep saying this. While it is probably fairly accurate statement, under most circumstances for Bitcoin mining it does not make financial sense to go for that extra 2-3% additional efficiency when you end up having to pay $100-200 more for the larger PSU just so that you can run it at 50% load.