The 500w is what they recommend you get for a complete system with said GPU, the card itself doesn't actually require 500w:/
You might want to get a larger PSU so you can use it when you want to upgrade later on.
Get a 80 plus certified PSU with safety features. In other words a quality one. I am personally fond of Corsair. Their 650w enthusiast is popular.
As to whether or not you should get some more ram depends whether or not, as has already been written, you're using a 64 bit OS.