Having multiple monitors isn't good eronomically IMO. I always prefer what I'm working on to be on direct center, and I don't really see the point of having other stuff that isn't direct center. I prefer to alt-tab.
I have a large monitor on an arm in the center flanked by monitors to either side. My main monitor just has a full-screen emacs. The other monitors are for peripheral information such as browsers, irc sessions, email, etc.
It's great for keeping API docs open while I'm working without leaving my editor.
Having the center monitor on an arm just lets me adjust it's height when I move between leaning forward, slouching, or sitting straight, standing, etc.
A single monitor is limiting as it requires more context switches as I swap out windows in my viewing area. It's much nicer to just leave all the important windows open in a nice grid that spans multiple displays.
I love 3 monitor setups, with the big one in the middle and two cheap small ones on either side. All my input (editing, browsing, etc.) happens in the big fancy middle window. Test logs on the left, and server logs on the right.
The only time I use the side monitors for anything else is if I'm following along with a screencast.
I just have found that if you want to read something, I prefer it to be on the center monitor anyways, so what's the point in having multiple monitors?
Plus, I prefer to work on my laptop screen, because it's more flexible - I can put my feet up, move around, change positions easier.
I have recently started using a laptop underneath a big honking monitor. It is fantastic. You get a second screen (down low) for stashing things like shells or firebug and looking at the big monitor (up high) forces good posture. Going back and forth feels more natural than looking left-right. Switched away from equal sized dual monitors. Never looking back.
