Hi,
I think CoD4 is different from the other two as its basically a shooter, lots of fun and its simular to all CoD games just set in modern times.
Both ArmA2 and Ofp2 havent been released yet and i dont think i would hold my breath either and i would expect them (if at all) fall 2008. At present you have a choice of either Operation Flash Point which although still played is out dated by ArmA, both are excellent games and i would liken them closer to a sim more than a shooter.
One thing to keep in mind is that both need a decent pc to run and Armed Assault still has its bugs and conflictions as its early days for the game but like Lock on its worth your time.
steve