If one monitor is not enough for you, you can always buy another one, if that will satisfy your needs. Both of them can be connected to the same computer if your graphic card supports that. There are many benefits of having at least two monitors:
1. You will have more than one desktop. This will give you more space to put all the necessary files you use. That can be quite handy if you work with many files like, for example, images.
2. Different monitors might be used for different purposes. One of them can be set to serve you as a working space (with emails etc) while another can be designated to serve as an entertainment station to play games, watch DVDs and listen to the music.
3. Many 3D games like Unreal Tournament or Microsoft�s Flight Simulator enable multiple monitors in order to enhance the gaming experience. Not many players have such a setup, but those who do, really appreciate it.
4. In business environments multiple monitors can be very handy as well, in order to allow employees to see more data on many monitors.
Ultimately, the decision whether to have a dual monitor setup is up to you, so you should take into account your needs.