PSUs are less efficient and will burn out sooner if you run them at full load.  My advise is to get yourself a gold or platinum certified PSU around 1000-1200w.  Overclocked at full tilt (350 MHz) you will pull apx 400w from the wall per miner...so your looking at ~800w for the pair. The conversion from 120V AC to 12V DC isn't perfectly efficient, so some energy is lost as heat(why PSUs have fans).  If you have a decent gold/plat PSU your looking at 87-92% efficient...so if your pulling 800w from the wall your miners are only consuming 696-736w...the other 104-64w are lost as heat. The less efficient PSUs (uncertified, bronze, silver) put out more heat, as the conversion is less efficient.  You also want to have some headroom on the PSU so your not pushing it at the max 24/7( = burnout & replacement costs).  I only use gold/platinum certified Corsair PSUs (wish I got $ for the free advertising) because they are pretty much tanks.  I'm running 4 Corsair RM1000 PSUs (gold cert.) and they have a 5 year warranty if I remember correctly.  They worked great on the S1s (I had 6 S1s overclocked on 3 PSUs...2 each), and the S3s.  You can do your own research, and this isn't exact ,but 80% of the rated load is normally approaching the point where efficiency starts to decline because of the heavy load/heat buildup.  You could also run an additional single blade(half an S3) if your not overclocking any of them...overclocked that would be using 1000w and would be incredably hard on any PSU.  My advise is pay the extra for a reliable strong PSU.  You also pay a little more for fully modular, but if you have alot of gear, it keeps clutter to a minimum....may not make a difference to you if your only going to ever have 2 miners, then modular doesn't matter so much.  Just remember...additional cables in the way reduces airflow and increases temps...maybe not to the point of making an impact...the rule holds true to life (ie...computer case cable management).  Good luck and happy mining.