Low. you can use FRAPS to gauge your framerate and adjust your settings accordingly but i'd just leave it on low tbh.
image sync just prevents screen tearing but i wouldn't bother toggling settings. COD4's really well optimized but (no offense) in terms of gaming, your playing it on a calculator so i wouldn't worry too much about settings, just set them low and have fun.
Also, there's a forum for gaming and a forum for macbooks. Either way, this is the wrong forum.