500 Watts is good, although 600 watts will be a nice elbow room especially when it comes to have a bit powerful fans... My old ATX power supply went belly-up (Ultra 400W) as it was iffy... And, I had to use Dell ATX power supply for a while now, so stressed beyond its rated power, 250 Watts while load is actually 400+ Watts (Phenom II Deneb build - I had to save up money and the likes - I do my works on it, like printed-circuit board CAD). I would like to replace it soon. Why PFC? It's better designed against sudden changes in current-drawing, like switching on Delta fans.