Why would you want to play with dual monitors? Having the center of your view split between the monitors isn't that fun. lol
Triple display for games is where you wanna go for multi-monitor gaming.
Supreme Commander will take advantage of having Dual Monitors. Not spanning the screen but you could have one monitor zoomed in on a battle, and the other zoomed out to manage whats going on.
There's a guy on an AV forum I check every now and then who's investing in a 3 projector setup for slight sims and racers. It sounds horribly expensive and hard to arrange, but would surely look awesome.