I cant agree with what cosmic is saying. My current rig can run singleplayer at full graphics and resolution with 4x aa. When i took the graphics card in to be checked, they gave me a 7800 as a spare. Using this, i got less than 20FPS on lowest settings (1024*68 resolution). If the graphics card cant keep up with the cpu it will dramatically reduce your performance.
Ive found overclocking to be only useful if you find out where your computer's bottleneck is and work on that. In your case that would most likely be the graphics card. Wit $400, i recommend learning to speak asian, going to MSY and buying a Gigabyte or Inno3D GTX260 ($300) and G.Skill-PI 4GB Kit (2x2G) DDR2 1066mhz ram ($100), and scrapping your current ram, which would probably be much slower. Theres no point keeping it cos 32bit OS's can only support around 3-3.5GB of ram anyway.