The company I work for has dual monitors for everyone in the office, and although I don't have any quantifiable data, I guarantee our productivity is incredibly higher than if each person only had one monitor. I'm writing from home right now where I only have one screen, and I definitely miss the second one. :( It just makes comparing things so much easier. When I used to have to alt-tab between pages, I can now have both pages open, one on both screen.
Please do yourself a favor and go out and buy a second monitor immediately. It'll be well worth the investment. Actually, I'm going right now to buy a second monitor for my computer at home! I can't wait!
Regards,
Koby