A 450+ Watt PSU (WITH 80%+ effeciency) from a good brand like Corsair/Antec/Coolermaster, etc
Important to remember two things
1. Efficiency - You dont actually get the promised wattage, if the thing says 500 watts, and has 75% efficiency then your getting 375 watts (75%) not 500 watts. So dont skimp on the PSU, pay the extra cash for a really good product, it'll be worth it in the long run
2. Do you plan on upgrading and getting a full blown graphics card? if so, you'll want to account for that
I have a GTX 570, and use a 600w Corsair, runs great