Wattage is not as important as the SPU maker. Go for a good brand like Corsair, Zalman or PCP&C. 450 watts from those makers will be enough for just about any setup. A cheap 500 watt PSU will produce only 250 watts max and blow when stressed beyond this point. It puts your components, home, and safety at risk!
I would recommend either of these, depending on how much money you want to spend: corsair 520w corsair 450w
