what you got now m8? and what size screen have you got? what budget?
cod4 has very low requirements: NVIDIA 6600 or better/ATI X1300 or better
a 8800GT would be an idea purchase
http://www.newegg.com/Product/Produc...82E16814143118 and should handle any game on max apart from crysis, unless u have a monitor over 22"