I personally find multiple monitors distracting if I don't need to follow lots of log output simultaneously. The 27" Dell U2711 [1] with 2560x1440 works perfectly for me, and is pretty much the only monitor with this resolution in an acceptable price range (~$800). On this screen there is enough space for Eclipse, two Android emulators, and part of a terminal showing the logs, for instance.

A 17" display with about 1920x1200 also just works for me. I find lower resolutions very hard to work with.

[1] http://www.anandtech.com/show/2922