Antec Earthwatts Ea-500 500w
 

500W is enough to power mostly all mid-range systems, the Antec's one has excellent efficiency rating, or so they claim:

"The EarthWatts product is Antec's attempt to address the specific needs of those looking for the most efficient power supply available. The EarthWatts is 80 Plus certified; meaning that it has 80% or better efficiency at 20%, 50% and 100% of the power supply's total capability. A typical upshot to a power supply that is more efficient is that, because less power is wasted as heat, slower, quieter fans can be used to cool the power supply unit

http://jonnyguru.com/review_details.php?id=51