I have used this PSU calculator from time to time, whilst not completely accurate it does provide an indicator to what you would need for specific components to run reliably with each other with a certain size PSU.

http://extreme.outervision.com/psucalculatorlite.jsp

Yours for instance comes back with recommended of 655Watts and Ideal PSU for 705Watts, so not too far from the truth tbh, seeing how like I said its not completely accurate.