You can only do gaming and general task (realistically) on dual monitors if you use windowed mode in games. Some of them have 'borderless window' modes, like Left 4 Dead 2, which will have the game full screen on one display, while being able to use the other monitor for other things, and not having to worry about context switching problems.
You can use two displays on one graphics card. It's a bit trickier using three displays on one card. You can't with NVidias afaik, but AMD's support three displays, usually using the two DVI outputs, and a displayport output at the back of the card.