I know that I have written about this concept a lot. Thus, rather than blathering on about how much a second monitor can increase your efficiency, I will point you to an excellent post from Laura Calloway, the PMA for the Alabama State Bar. She begins:
If you’re not already doing so, one of the most productive (and cost effective) technology changes that you can make is to add a second monitor to your computer. Adding a second monitor is inexpensive, (generally under $200 for the monitor and an additional video card, if you need one). If you already have an extra one lying around the office, so much the better. And unlike lots of other technology upgrades, adding a second monitor won’t tie up a whole afternoon – or longer – and make you want to pull your hair out.
Go here to read her entire post.
I resisted the 2nd monitor for awhile, but now that I finally got over the hump, I have to say it’s a huge help.
I still work mostly on my primary monitor, but having the 2nd definitely saves me a lot of flipping under open apps.