The 560ti is an older generation GPU which requires alot more power than the 660ti. Its one of the reason why the 560ti has two PCI-e connectors on the card instead of one like the 660ti.
For the 560ti you will need at least a 650w PSU if you plan on running the 560ti at stock speed, however if you overclock it then you will need a 750w power supply. You would need a powersupply that has 4 PCI-e connector plugs to power both your 560ti. Make sure it is a good brand as well! I usually go with corsair, seasonic, or OCZ
System in IDLE = 201W
System Wattage with GPU in FULL Stress = 449W
These are the specs for the 560ti in SLI. As you can see your GPU running SLI would draw 449w alone at full GPU usage. Add in the proccessor, ram, cd drivers, and etc and you would need 600+W to run your entire rig stably in SLI .