I think that many people believe that the power supplies that come with a case are 2nd rate. Often they are cheap and not well made, although some (by companies like Antec) are probably decent. I think that most custom builders would prefer a brand name PSU (Antec, Fortron, Enermax) to go with the system. 500 Watts should be more than adequate for your system. You could try the built in power supply and see how it works for you. If you are having major problems with instability or not booting up properly, then consider a brand name PSU.