Ahoy,
Tried on two machines, MBP 2.5GHz, 2GB, 512mb 8600m - It runs smooth as anything on default settings it chooses, no noticeable slowdown in either single player or multiplayer, if it can run crysis on medium (well) which it can, it can run CoD4
On a 2.4GHz, 2GB, 256 8600m - slower loading (only noticable when compared directly) but it still runs fine, to be honest, I can't really tell much difference between them, the extra memory on the 8600 seems to make no difference at all.
This will help too -
Call Of Duty 4 on MBP? - Mac Forums