I can't speak for crysis warhead(never played it), but CoD4 has ALOT of replayability. There's 55 ranks and when you reach a new rank, most of the time it will unlock a new gun to use. On top of that, each gun has different attachments to unlock based on certain achievements ( get x kills with this gun, get x headshots with that gun). There's also kill streak rewards (3 kills in row=UAV, 5=airstrike, 7=helicopter). <---I think those are right; I've been playing MW2 and you can assign different rewards for different kill streaks.