The 3200 will give you no discernible boost in performance and probably cause you more issues. The mixed-mode crossfire setup has not been terribly stable. You are better off just waiting for the second 4670.
if you have the 4670 1Gb, then yes, more than likely you will be able to play GTA4 and MW2. the 4670 1gb is better than a standard 3870 512mb