Yes, you'll be fine with that setup, Quad Core at 3.24 is FAST enough to play any games with decent setting (in this case GTX470). Well, GTX470 is a little faster than HD5850, but produce more heat. So as long as you have good airflow inside your case then go for that card.
Errrr, better than what? the GTX470? Go for the HD5850..... it's cheaper, offers nearly the same performance and works a lot less hotter Save some money and save the planet....
Yeah, any mid-high spec 5xxx series will be great.
470 is better than 5850 http://www.anandtech.com/bench/Product/164?vs=160 Power wont be a problem.Just get good airflow in your case as wa1 said.If you are still concerned about power get the GTX 470 SOC from gigabyte.It performs almost as good as the 5870.