I always buy a much larger psu than is needed. It is a false economy to buy 500/600w entrance level psu's. My reasons are:
* At some time or other you are going to want to add extra components to your build. Your an enthusiast right?
* Larger psu's work less hard delivering 500watts than 500w psu's do - fact. Always allow a healthy headroom so the components in them are not allways working flatout.
* A larger psu generate less heat comparably yet only consumes marginally more power than a smaller one at that rated power.
* You get more connectors and generally, the build quality is better with better (bigger) cooling fans. Large fans turn slowly, use less power and generate less noise.
* Always pick a psu with a 3 year warranty. Choose carefully, buy once then you get 3 years peace of mind without having to worry about replacing or upgrading. Don't forget; they manufacturer would not give a 3 year warranty if they thought they would get a load of returns. As for one year, well some don't even make that.
* Pick a psu that has interchangeable lead sets. Then you change the lead not the psu when the technology changes. Most high end manufacturers will keep the same socket arrangements.
Just remember: A psu is THE most important component in your PC. It supplies power to all your expensive components. If it is not regulated, or doesn't have decent over/under voltage/current protection, and thermal protection then don't buy it.
Personally, I would recommend not buying anything under 800w and for all my PC's around the house (4!) they all have KW psu's and over.