usually its video drivers that causes lag on even high end systems , for instance Test Drive Unlimited on my pc didnt work until i downgraded my video driver to version 7.7, since 7.11 gives me issues aswell as 7.10,7.9.....
is it all the time? cause i have that card and i can play cod4 on highest settings but i do notice that from time to time in big fire fights that I do get some fps lag.
but it shouldn't be 100% cpu usage with your cpu...