I have dual monitors, but they are not great for gaming. You can use a program called SOFTTH:http://www.kegetys.net/SoftTH/
This will allow you to play with dual monitors on any direct x 9 game. Pretty much the only thing I use it for (and this is totally awesome) is split screen. It great having each player get their own screen for portal 2. But, if you want to play single player, the crosshairs are right in the middle of the screen. For single player gaming, 3 monitors are awesome because you dont have a split in the middle of the screen.
hope that helps