A very high quality (e.g. a Seasonic) 500W PSU may very well have 40A (480W) available on its +12V rails, but even a lesser (but still good) 500W PSU should have 34A-36A available for +12V.
Based on my arbitrary entries for # of drives, # of fans, etc. the PSU calculator at http://www.extreme.outervision.com/PSUEngine
suggests that you only need a 416W PSU. Your Odin may actually be enough, although I would agree that it is not a very good PSU. A quality modern PSU has full range active PFC (no little voltage switch) and some level of 80+ certification for efficiency. My own rigs use only Seasonic or Antec, but Corsair, XFX, and Enermax/LEPA are also good.