Would go a little bit bigger than that. I have a 750 watt with my 560ti (2 hard drives in puta). I -think- I did the maths when I purchased it and it allowed me a little bit of room for another couple of hard drives if needed. Have a look at some other manuf cards of similar specs and they should list out power requirements.
560ti
"Requirements
Minimum of a 500 power supply.
(Minimum recommended power supply with +12 Volt current rating of 30 Amps.)
Two available 6-pin PCI-E power dongles"