I have just got a second monitor which I've been using for the last week or so - my main monitor runs off DVI so this just plugs into the VGA port of my graphics card. It works well although it's hardly a very elaborate setup, I tend to like putting a wing view on it. Be warned it does drag the frames down at busy airports - I wouldn't fly into LHR
with the second view with full AI
at sunset or anything, I tend to have it on during the cruise.
I have more used it for things like ServInfo when flying online and FSAcars to save me exiting the program all the time which obviously isn't so heavy on the frame rates. Some people have some very elaborate multi monitor setups to create a "cockpit panorama" but I'd imagine they do have a large impact on the performance.
Once you have tasted flight, you will walk the earth with your eyes turned forever skyward