I recently switched from three 19" monitors to two 27" monitors (I'm a php / javascript / python developer). I assumed that having the same horizontal resolution with extra vertical resolution would add to my productivity.
With three monitors I had three contexts in which I merely needed to glance in a direction. Almost like doing an "alt+tab" in my mind. If I wanted to check my current code in or run unit tests, look at my left monitor and type. If I wanted to read or write code, look at my center monitor and type. If I wanted to test output or find a reference, look at right monitor and type. *
With two monitors (regardless of resolution), I have one less context. I actually have to think about window placement. Those seconds add up. More importantly the cost of switching context adds up as I get deeper into what I'm working on. If something was important, I maximized it to a screen and that's all that screen was. That is what it meant to look in that direction. Now I maximize a window and... well a 27" window is pretty ridiculous unless your work involves graphics. There's less permanence and hence more to remember.
Save money on the resolution and get yourself as many decent-sized monitors as your desk / system can handle. I assume it's possible to have too many monitors, but I feel I would have done better with three slightly larger monitors (or just a fourth monitor) than I am with two that can hold more pixels than the three. I plan to find out very soon.
* By "look and type", I mean tap my mouse in that direction and click to activate the window and then type.