Doing some small upgrades.. what will improve my OA performance?

My PC,

AMD Athlon II X3 445

ASUS M4N68T-M-V2

PCS AMD HD 5770 1GB

I like playing on high player count servers with heavy missions(Example: Warfare BE, PRmod), what will significantly improve my performance?

ASUS 6870 or a AMD Phenom II X6 1055T? Don't suggest Intel I'm on a budget. And, will OA make use of all of my cores like BF3 does?

Please search before posting:

- Will my PC Run this? What CPU/GPU to get? What settings? System Specifications.