Unless, of course, he doesn't want two computers...Main Mac: MacBook
Gaming: E4300 (easily overclocks to 2.8GHz and above), 7900GS (easily overclocks), etc.
Monitor: 20" IPS LCD
This way you have a fast Mac, a laptop, and a gaming PC.
All of this can be acquired for far less than the price of an upgraded 24" iMac or an entry Mac Pro.
Are you running the Universal verison of the game?COD2 is still a very demanding game. On the 20" iMac with the X1600, you can play it at 1024x768 with medium settings, but the frame rate will drop below 20 in many scenes. Still, it's playable.