Definitely Scourge of War Gettysburg
http://www.scourgeofwar.com/index.shtml
A gameplay video from you tube
Its like Total War if you've ever played that, good graphics for a strategy game, you can zoom in on individual troops, move around regiment and brigades, but a lot more realistic than Total War. You can command anything from a brigade to an army and play out historical scenarios of Gettysburg. There are also other scenarios of Chancellorsville, Antietam, Brandy Station, and a what if of Pipes Creek.
As mentioned above there is an ACW mod for Empire Total War and a mod for Napoleon Total War too. Those are both great games but the AI isn't too smart and even though the graphics are excellent, the bad AI can make the gameplay not so realistic. Although, both games have multiplayer. Also you need a pretty good gaming PC to play Total War. The most recent game in the series is Shogun 2 TW and in September their coming out with Rome 2 TW.