For that build 500w is plenty - just ensure you get a quality model from XFX, Seasonic, Antec or Corsair; all great PSU brands.
Just taking a look at your build, it looks good, but I seriously recommend getting the WD Green as your main drive, as it's known for being a very slow drive due to its energy saving capabilities - go for a Seagate Barracuda or WD Blue/Black instead.