It's a pretty difficult task choosing the perfect power supply. You can't have too much or too little. This article helped me greatly and I hope it does for you. It takes actual draw in power and even gives you the cost of running your card for a year.
http://www.tomshardware.com/reviews/geforce-radeon-power,2122-4.html
http://www.tomshardware.com/reviews/geforce-radeon-power,2122-4.html