Mikeffer, on 20 December 2012 - 11:49, said:
Remember, a quality 300w will be more stable and just plain better than a cheaper 500w
This can't be stated enough.
Sizing a power supply and choosing a quality unit are the MOST important things to do when building a computer. Think about it - every other component in your system connects to the power supply - every single one. And a bad power supply can cause all kinds of havoc with your components.
The most common mistake people make is choosing a power supply that is over-rated for their needs.
This is bad for several reasons:
1) Running a power supply at half it's rated capacity is very inefficient. On the flip side, running it at near full capacity for extended periods is also bad. Aim for somewhere in the 70-90% range (at load) depending on how much headroom you want. You should look for units that are 80+ certified. Though there are issues with the 80+ certification (which you can Google for more info), this certification from a quality manufacturer shows they've put the development time and effort into producing a quality power supply that will maintain certain efficiencies across its load range.
2) Often someone will choose a supply that is over-rated for their needs and wind up buying a cheaper model. Now not only do they have an inefficient supply, they have also purchased one made with cheaper components that, if they're lucky, will simply die, if not, could take other components with it when it does die.
Use a power supply calculator to rough out the size of supply you need given all the components you have. You may not be able to account for everything, or choose the exact components, so add maybe 50W to the calculated value. The biggest factors to account for are going to be the CPU and GPU.
Then buy a quality unit at that value (Seasonic, Corsair, PC Power & Cooling, Enermax) and don't spend the same or less for a higher rated, but cheaply built unit.