a power supply is a power supply really, performance will not change even with a bad quality inefficient supply of power, as long as their is enough power being supplied to the system it will function the same regardless. But a bad power supply can overload easier or under volt/over volt the system when being stressed. Thats why its imperative to get a quality psu, in your case a good 500w would be fine but 600w gives you some upgradability.