Your CPU is the bottleneck. A friend had a gateway with an AMD 64 x2 5000 and an ATI 3870. He was getting over 100FPS on COD4. He couldnt overclock his bios, but he did OC using the OC software that came with the Gateway. He had his at 2.8G. Find you a good software OC app (nTune, Easy Tune, etc.) and try it. Dont go crazy, because soft OCs don't manage your voltage very well and it could fry your hardware. OC in small bits and test with prime 95 and watch your temps and voltages.
Later my buddy did go with a AMD 6000 and now he is over 120FPS consistantly in COD4. He soft OC'd it to 3.2G.
With the 4850(which is much better than my buddy's 3870) you should be hitting 100FPS in COD4.
There are options for you. Carefully try a soft OC, OC in small bits and test. If that doesnt get you over 80fps, get a 6000. Since you cant OC your bios, you dont need a black edition. Just get the regular 6000.
What are the rest of your specs; ram, etc. RAM may help.