Somethings not right with your setup then.
Why do I say this?
I tried as an experiment to install COD4 on my #2 rig.
Its a p4 Sct 423 (The old one) 1.7ghz which is well below the system requirements on the box, 512MB RAM and an aged VIA Chipset, the systems only saving grace is its 128MB 6800LE AGP card, hardly cutting edge.
I spent a good few hours on-line with it over the weekend playing in full (50 player) servers.
Obviously the graphics were set to low and I did not try to increase the settings but the game ran absolutely fine which very much surprised me.
That said, its a relatively old game engine, so why would it run poorly?