470 gtx or 5850

Hello,

I'm building a system with phenom ii x4 3.4 ghz, 4 gb ram, but i just need to know which is better so i can finalize my mobo

should i go for the 470 gtx or 5850? I need a card under 300 dollars that has stellar performance and one which can run wow-cataclysm at max fps, and will last for future games.

i need to keep this system around 1-1.2k dollars. also why are the i7 so much more expensive? is there a way I can build a computer around 1-1.2k with an i7 and a good graphics card? please help would be appreciated. i have a monitor and windows 7 i can get from my school.