If your looking for a good FOS title deffinetly check out squad it's Arma on steroids and runs fairly well on most systems. Combat is crisp and realistic and focususes on platoons(usually 5 squads/platoons per team) that work to achieve a goal like making an FOB, making a mortar site, assault teams. Etc.
The game has a lot to offer and it's probably the best FPS game on the platform if your into more realistic play.