yep also your likely to be getting a slight bottleneck with that gpu on that cpu... your at the very limit of what an amd cpu can handle... if your using a 1920/1080 or bigger screen of 23"/24" then use x4 aa maxium. any more and you wont see it but you will get a serious fps hit...
the higher the dpi and the smaller the screen the better because you wont need aa to hide the jaggies.
on a 21"screen you can get away with x2 aa if its 1920/1080 because the pixel pitch of the screen is so small.
adversly if you were running on a 19" 1280/1024 screen i would say use x4 -x8 aa as the screens dpixel pitch is pretty big and will need more blending to get edges looking smooth.