On Win7, Windows key+left/right snaps windows to half a screen, and windows key+up snaps it to a whole screen. Combine this with two screens and you can fit a lot more windows on the screen with minimal effort. Likewise, a lot of full-screen games and movies and such only take up half your real estate if you have two screens - right now, I'm playing Railroad Tycoon on one screen and chatting on Facebook with the other.

Also, who makes a 40+" screen that's 3000+ pixels wide for anything like the cost of two normal monitors? (I mean, they may exist, it's a long time since I've gone shopping, but it seems unlikely)

I have one monitor aligned horizontally and another aligned vertically, and I feel this works better than one large monitor or a monitor that swivels between the two alignments. I think part of this is because with two monitors, it's possible to rotate them relative to each other (ideally, I think, you would want a spherical / parabolic / hyperbolic monitor and this helps approximate that), but it might be that if I had hotkey shortcuts to easily throw windows where I wanted them on a single large monitor (like xmonad) I would be able to get most of the benefit.

If you have a laptop you can't just grow the primary.

The one study supporting one big screen is of bad quality, the studies supporting two screens are of medium quality. The former study was done by Apple, it's small and people were doing only one task (Spreadsheets); the latter ones had a bigger sample and people did various tasks. A survey over the productivity-blogosphere revealed people are mostly in favor of two medium screens over one huge screen. Bill Gates has multiple medium-sized screens, so does his close subordinates.

I have two bigish ones (23'', 1920x1080), just in case. I consider this was the most amount of utility per dollar I got in the last 2 years, excluding money spent on relationships. I intend to buy a third bigger one (33'') soon.

usually it's cheaper per amount of screen real estate, but also it works a lot better for simultanously doing things that prefer to take up a full "screen" like games.

I will once again plug the Kinesis Advantage keyboards

I use the Kinesis Freestyle 2 for the same reason. It was worth every penny and I bought a second one for work, too.

Yah, +1 for kinesis keyboard for ergonomics. I get wrist pain when I use my laptop (don't use my Kinesis) for about a week; switching back to it tends to quickly relieve the pain.

That said, measuring pixels is also probably not really looking at what's really important here.

Pixels seems to almost definitely be the right measure, here- basically, you want to have N windows open with enough information in all of them. For most tasks, N is greater than 2, but once you have enough windows extra space is just wasted. (One of my former labmates had four monitors, which generally had Matlab open in one window, the docs in another, his system monitor in a third, and nothing in the fourth- so he probably would have been fine with two monitors most of the time- but he did sometimes do something involved enough that he needed all four. For someone working on code where lots of different things all matter, having lots of monitors seems useful.)

There is probably an upper bound on the number of monitors that one can comfortably and actively use, though I do not know how high it is. I've noticed that with more monitors that one is more likely to place static informational windows (social media, system monitoring, email/instant messaging) that aren't really necessary as an immediate concern. I'm not sure if it can be exploited to put valuable information there, or if one can train themselves to actively use it for work.

Managing windows and workflows with more monitors is also quite a bit more difficult than it is with a single screen. I've had some success at reducing wasted space by using a tiling WM, but having to learn key combinations to navigate is tedious.