From a productivity standpoint, I believe you go up on average 60% just by adding a second monitor (heard the value somewhere, too lazy to look up a reference).
From a gaming standpoint, it depends on the game I guess. An FPS game with just two monitors would have your targeting reticule and main area for aiming split in half between two monitors. For other games, such as SupCom, you're able to have two views of whatever you want -- one can be used as just a map, or you can zoom in on that monitor to another portion of the battle while your main monitor keeps track of your base. Give it a shot, see what works for you.
Obviously you'll need the graphics horsepower to run both of them simultaneously.