well using sli speeds up your computer, video cards are actually general processing units, so you will be able to encode videos faster your games will be faster, as far as monitors go not much changes except you can have more that one monitor, i personally am getting crossfire ati cards soon, so if you have the money i would go for 2, go for 3 if you are serious about it.
For your monitor and resolution, it will increase FPS, and some games might allow you to have better performance. However a single card will do practically anything you need it to anyways.