I recommend a modular design power supply made by Rosewill or Corsair.
1 GTX 660 TI would preform better than two GTX 560 TI's in SLI.
I would recommend ASUS as well since they have a copper heat though out the whole GPu. I don't know, just a suggestion. I tried Have a nice day.
For a system using two GeForce GTX 560 Ti graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 700 Watt or greater power supply that has a combined +12 Volt continuous current rating of 44 Amps or greater and that has at least four 6-pin PCI Express supplementary power connectors. Source: http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=w...
Here is Guru3D's power supply recommendation:
GeForce GTX 560 Ti
On your average system the card requires you to have a 500 Watt power supply unit.
if i were you i would get one of the higher quality psu's on bigcycos list not that rosewill hive is not nice but if there is a part you do not want to go cheap on it is the power supply if you do not get a nice one and let say there is a power outage a cheaper power supply will give out on you and can eve ruin other parts in your pc and you should give yourself a little more head room for upgrades in the future.