you can run most games on a 2 monitor setup. but its not really that good. your center is between the 2 screens. with 3 your center is a screen rather than being off to 1 side.
games like x3 will allow 2 monitors. 1 for the main window the other for the control-panels.
i agree with hexit. If you're thinking of playing with multiple monitors 3 monitors is something i would recommend than 2 monitors. But for that you'll need an amd/ati card.