Not bashing your thread, but that's a little steep in price for an outdated setup such as that, especially with a low end AMD. Have you upgraded the graphics card in it? Because at those specs you would really have to run COD4 on super low settings to get any kind of decent frame rate.
Also what's the model # on the computer, a lot of people would want to look it up to see what kind of board is in it for upgradability.