Dual monitors are nice. Lots of desktop space. Unfortunately, during normal daily use they can cause some strange problems related to window positioning (maybe thats just an issue with my card...) But if you like to multi-task, especially if you want some sort of video/tv going on at the same time as anything BESIDES games, it is almost indispensible, not to mention all your friends will ooh and aaah when they see it.
For gaming, unless you have a much more powerful videocard than mine (FX5200) it'll be pointless. If you want to game with multiple monitors, I recommend only odd numbers of screens (1, 3, 5, etc) and only with latest generation videocards. That way you can see down your nose a bit better without the big space between the monitors and still get marginally acceptable framerates.