In all games the framerate will rise and fall depending on what the graphics card has got to render. That said, 30-60 is quite respectable for your system. My own system is considerably more powerful (GTX680 with i5-3570K @ 4GHz), runs around 60-85fps and tops out at about 120fps. Over time it will get better as the graphics drivers are updated and the game is patched.
You've also got to remember that CoD4 runs on an older/lighter engine than Black Ops 2.
The most important question is, does is affect your gameplay?