You need a 400w power supply, and a decent one at that. Go for a corsair builder 430w. Its only $50. (http://www.amazon.com/Corsair-430-Watt-Certified-Compatible-Platforms/dp/B004W2T2TM)
If you want to keep your current PSU, go for a 6670. Keep in mind that it will be able to run old games on low settings, don't even bother trying to play BF3, skyrim crysis or anything big on it.