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.
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.