To be honest, with the system you are building 500 watts should be plenty
I'm running off a 550 watt, and I'm running a quad core, 8800 card, etc
Some PSU's do not produce the power they state though, which is why its always good going for a well known brand and to do research before hand.
A non branded PSU for example might say it produces 600 watts, and maybe it does unloaded, but if you then start loading it, that 600 watts can easily drop to 300 and less, using the rest to increase heat levels. Not something you want.
So getting one with a good efficency rating (you know you'll get most of the power when it counts, and not as heat or haha, light output) is a good move.