Until recently my wife was playing World of Warcraft on an old 17" 4:3 flatscreen monitor, while I had changed to a 22" 16:9 screen a year ago. Having seen how much nicer it is to play on a broader screen, she now also wanted one, so I got her a very nice HP 22" monitor. That left us with one excess screen, and I was wondering what to do with it. So I tried to connect the 17" screen as second screen on my computer, and that worked like a charm.
What you need for this is a graphics card with at least 2 exits, but most modern cards have that now. My card being a GeForce, I used Nvidia's nView software to set up the second screen, which was very easy. Then I just needed to switch World of Warcraft from fullscreen mode to windowed mode, with the WoW "window" taking up my main screen completely. And now I have WoW on the main screen and a second screen with a desktop on the side screen. Or, if WoW isn't running, I have two desktops.
When I move the mouse pointer to the right border of my main screen, he automatically jumps to the second screen now. I can even drag and drop windows that way. So my usual setup now is to have whatever game I'm playing on the main screen, and a browser window on the second screen. No more alt-tabbing and missing things happening in WoW while searching some WoW database for information. Very nice!
Theoretically I could even play World of Warcraft stretched over 2 monitors. In my case that might be a bit difficult, the two screens not having the same native resolution. In any case that setup has the huge disadvantage that your character is in "the middle" of your dual screen, so he kind of gets ripped into two halves with a gap between. It is, however, possible to use 3 screens, if you have for example two graphics cards linked. Then your character is in the middle, and you get an extremely broad 48:9 view for best possible spatial awareness. But then you need an addon to completely rearrange your user interface, or you end up with the chat on the left screen, your character on the middle screen, and your mini-map and right action bars on the right screen, which probably is unwieldy. I don't think I need that much space for WoW.