I'd do a 650 watter for a single card system since anything smaller costs of quality construction maybe $5 less. A 750 watter if not overclocking two cards should be fine. With serious OC on two GPU's and CPU, I'd use an 850 watter.
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.
Sometimes, a PSU that doesn't have two PCIE power connectors doesn't have enough amperage on its +12V rail(s) to power more than one. Your Odin may be in that category, regardless of what's on its label. Using adapters could be a fast and painful way to discover that your PSU isn't up to the job.