I have always loved multi-monitor set-up. There has been evidence that having multiple monitors with the appropiate screen size can make people more productivem, specially programmers. A lot of people (including Randy Pausch, R.I.P) supports this idea.
I personally I believed in the same and since 2006 I use multimonitor setup in my home. I haven't measured how more "productive" I am (at home I don't do a lot of programming) However, I can say it is definetly a pleasure to work this way.
I always wondered if there was some scientfically based evience of this, However I did not bother to search for them. Today I found a question on Skeptics (a StackOverflow backed site) about the same topic. And a lot of scientific literature is referenced. You can see them on the top answers to this topics.
NOT my setup :'(.
So, it looks like may help having multiple monitors, at least it will save a little bit of time. From my experience, and when programming, generally I like to have the reference manual or examples of what I am particulary doing. When using one monitor I have to switch between windows in order to get the info I need. If I have two or more displays I am able to put the info side to side without wasting my time in window management.
Sadly, in my current job I don't have access to a second display, the main reason is that I only posses a single video card with a single output (VGA). So, it is imposible to have a second independent display. However, there are some alternatives. One many usb-based video card (pretty much of all them based in DisplayLink technology) that cost arround 40-80 USD. They are sometimes laggy and does not wokr for video. But, for a programmers life which is pretty much text based or web surfing I think it will be enough.
In my case I've got the lugable UGA-125 USB 2.0 to VGA/DVI/HDMI . I found an a spare monitor that no one was using and moved it to my place. The installation was easy under windows (my job official OS).
After some time using two monitors I feel totally happy and although I don't have the data, I that feel I waste less time while writing code, and well, doing other not so enjoyable tasks :-).