I play games on 1 monitor and have something else on the 2nd, movies, TV ect.
Than that shouldn't be a problem.
Playing games on 1 monitor is fine and with the right setup (an AMD Eyefinity card or SLI Nvidia cards) there's more support for 3 screen gaming.
Playing games on 2 screens is a bit more complicated I think.
You just have to decide what card you're going to buy now.