As with what everyone else says, it's hard to quantify a 5-10 or even 20% gain in productivity with dual monitors. But being that I have had duals for at least 2 years, I would not go back. When I work on a laptop or someone else's single monitor I keep looking for the 2nd monitor.
AutoCad utilizes the dual better then Microstation because Microstation does not let you take toolbars/pallettes outside of the main window like AutoCad does.
If you do a lot of file management stuff, the 2nd monitor helps to have two windows with file directories.
I do work off of PDF's, Images and other types of reference files and having the reference file open on one monitor and work on the other pays for itself very quickly. You don't need to print it out and it's right there for you to look at, zoom, pan etc...
The downside, is that it is more expensive. A video card that can handle two monitors and then two monitors. To save money, use an older CRT as the second. Even a 15" will make a difference...
Pieter