Two recommendations if you like realistic FPS games:
Day of Infamy. Even for the single player, practice matches against AI are great fun! Extremely atmospheric and hard (no gunsight) you have to use the weapons sights or guess! Seeing the AI use a flamethrower when attacking a dug out
I wish I had realised months ago as I have been hankering for a Day of Defeat replacement for ages. Only been playing recently and the memorable moments, dodging/hiding behind trees as bullets whip around or hit the tree stump. Witnessing a grenade land by but not go off, all heart stopping moments. Yet to try online as I am enjoying playing offline mode currently although going by feedback, it sadly suffers from quiet servers.
Arma 2 has mods for WW2. If you like realism, it is superb single player. Multiplayer is a mixed bag; co-op with a mate is awesome and one of the best gaming experiences. You can use the Mission Editor to build your own battlefield - the only limiting factor is your imagination. The AI has its moments but to be honest given the scope of the sim, I think this is part of the fun!