I have worked on computer that had dual monitors and quite frankly, I don't see the benefit, right off the bat.
Maybe a single monitor on a computer that isn't set up properly against a dual monitor on a computer that is set up properly will show better results, but the emphasis should be seen on the setup of the computer applications.
I don't have to minimize Autocad for ANYTHING. Why would you? Say I did, if I wanted to check another application... so what... it's not like you can type an email AND work in Autocad if you have dual monitors... dual monitors doesn't mean you can somehow suddenly provide dual input.
There's nothing a dual monitor can provide me that I don't have available to me on a regular monitor... web watching as Maverick mentioned... not important, but I do it, so I should consider it.... so... as far as web loading, a simply alt+tab and F5 tap isn't time consuming... especially considering that just because you have two monitors, it doesn't mean you still have to switch to the browser window and hit 'refresh' (unless you have an autorefresh addon activated, which can happen to either system)
As for email... I have that window set to popup any email notifications so i can see who it's from and then I can visually ((while working in cad) address whether Outlook needs my attention or not.
What else is there that dual monitors benefit, that a single cannot? Working space? Putting your tool pallettes, toolbars, buttons, etc on one? Why? I don't use buttons, and I only 2 rows of toolbars up... one row for the layer toolbar next to the workspace toolbar for quick switching... and one that has the 'new, open, save' etc next to a few random toolbars. Other than that everything is on tool pallettes on the left or right that AUTO Hide, leaving me with an estimate..1800x1100 pixel-area of working space, speaking conservatively.
What can a dual monitor offer me that single can't? My only guess is that you can have an excel spreadsheet up in one monitor and read it while inputting info into cad. I never do that, but I can see it's benefit... but when you can simply print the spreadsheet and have it sitting in front of your keyboard, or on a standing clipboard, I don't think I would have the gall to go to my boss and say "I want a brand new lcd monitor to replace this $8 clipboard" (although I don't have that $8 clipboard... I keep whatever sheet I'm working from between the keyboard and myself, on the desk. No probs there)
(edit) I know I sound of very strong opinion in those statements, but it just comes from long deliberation on the subject and having read quite a bit about the subject and I haven't found anything that tells me that it would be a good idea. I'm VERY open to someone showing me specific proof. I don't believe blanket statements of "40% improvement" because it doesn't break down WHAT improved... tell me HOW it improved or you're just bluffing, as far as I'm concerned.(/edit)