Warfare(CTI) and Domination mode, these two mode were the multiplayer mode of Arma2:OA Combined arms I used to play with friends.
Had some awesome experience, sharing 2 seats chopper online.
I'm not sure buying Arma2 and DLC's at this point is a clever option, as Arma3 is on the horizon though.
On the horizon meaning Q4 2012, that's quite a while away. Even when it's out, it will always be better after a month or two and the patches that come after that.
I learned not to wait for anything on the horizon after waiting years for cliffs of dover!
Now TOH will be work with ARMA2 content, it's going to be a great long term game.