None, the problem is not your computer, it's the game. A 9800GT and quad 2.8 is more than adequate for all games that, like Arma 2, were released more than a year ago. Of course, if you want to dig in your wallet to compensate for incompetent programming, be my guest. With your specs you can play a game like The Hunter with 50fps and have ten times better looking landscapes:
[iG]http://i50.tinypic.com/nbdkpj.jpg[/img]
[iG]http://i47.tinypic.com/eueo0x.jpg[/img]
Just my 2 cents.