I am currently running the XFX Nvidia 512mb 8800gt, but thats out of your given price range. I would try to at least go with an 8series card for COD4.
By the way, i have that game to. It is simply amazing!!!
Newegg is selling a XFX 8600gt 256mb card for 127.99 with free 3day shipping.
http://www.newegg.com/Product/Produc...82E16814150229
Then the XFX 8800gt 512mb card is going for only $269 ****Best bang for your buck*** Will run most all games at very high frame rates...except Crysis of course
http://www.newegg.com/Product/Produc...82E16814150252