500w easy with no GPU overclocking, but personality I would go to 600~650w, just for some headroom and less stress on the PSU's components.
Just be sure to buy from a reputable OEM company, I perfer Seasonic products. Other good brands are Antec, Corsair, Thermaltake, Silverstone, NZXT. You may note I didn't list Cooler Master/OCZ, as I've seen numerous cases corner cutting with alot of their models.
A very high quality PSU that's highly efficient:
http://www.newegg.com/Product/Produc...82E16817151088
