My main system is 7360 x 1600--four 20" in portrait mode and a single 30" in the middle.
Obviously, I recommend at much monitor as you can afford (and have room for). :-)
There's several different parts to typical development workflow--Ideally you would have enough room for some documentation / Google searches, room for some source control / local workspace browsing/searching, root for dealing with your bug reports and lists, room for 2-4 source files, and room for viewing your product and debugger.
How much concurrency you need of these views dictates the weight of physical monitors vs. virtual screens.
I've used a minimum of 3 screens for about 8 years now--I highly recommend trying out massive multi-monitor systems now that LCDs are so cheap to buy and operate.