Short answer, no. I played this game mostly on the PC but an 128mb ram video card and 1 gb ram will make for a basic gaming experience - it shouldn't crash but you will have to choose between lower graphics settings or lower frame rates.
Whether for CoD4 or general use, I'd up that RAM as far as you can from someone like crucial.com . I think your machine is prob max 3gb, so just buy a 2gb board. If its one of the newer silver iMacs you can fit it yourself easily. That will make CoD4 faster though you may still be limited by the graphics card. The new ATI cards do more with less ram than their Nvidia counterparts but that HD2400 is not a brilliant gaming card.
For multiplayer, i would say you'd have more not less of a problem. Multiplayer is buggier anyway, more prone to random lag, and if your machine is also struggling you won't do brilliantly. Its a twitchy kind of game so seeing things quickly and reacting to them quickly are important.
If there is a demo, why not try that? It will give you a better idea. As I said I'd up the ram anyway for general use (and certainly for gaming) and that may make CoD4 play great, but I couldn't guarantee it.
