About three months ago, I went dual screen. That is, I switched from a single computer monitor to a dual screen monitor.
I had read that a dual screen monitor can increase productivity by 50%. While I don't believe this statistic, the dual screen monitor definitely saves me 10 - 15 minutes/day.
The dual screen is really helpful if I want to comment on website, for instance. I'm able to have my email open in one window and my Internet browser in the other, so I don't need to constantly toggle back and forth between screens.
I guess this is the test of whether a dual screen monitor is right for you -- if you find yourself constantly toggling between windows, it's probably time to go dual screen.
From a financial perspective, it seems very straightforward. Let's assume 15 minutes of time savings per day and a $50,000 salary (and a 2,000 hour work year). That equals 62.5 hours saved per year, which is a savings of $1562.50. Since a dual screen monitor often costs only half of this, it's a great investment for most people.