It really really depends on what you're doing with your computer.
When I'm working on code, I find that one large monitor is as good as dual monitors, but not necessarily better - I just need lots of real estate for the code, output window, ancillary windows, reference websites, etc. I generally don't keep email and other things open in my line of sight as they're simply a distraction. (They're open, but put away until I need them)
However, when I'm working on circuit design, graphic design, etc then having dual or triple monitors where the 'main' display is larger than the others is absolutely essential. The main drawing surface takes up the majority of my workspace, and must be on a single monitor. I then need reference materials on the side and the fact is that I'm more productive if I can look to the side for needed info (a spreadsheet with a list of issues, a client's email with notes on the design, a datasheet, etc) without disrupting my main workspace.
I know that many programmers are happier with a single 30" than dual 24" because there's nearly as much realestate, and less head movement.
I'm a resolution junky, though, and I can't stand not having several sheets worth of code in front of me when I'm programming, so a vertical resolution of 1200 or more is essential to how I work. Same thing with design - I can see an 11x17 area in full size on a 24" screen at 94dpi (good enough that 6pt text is legible - as it is in print). I can't do that with a 22" monitor, and panning is just not as productive. (Kill the scroll bar!)
So... the real answer is it depends.
Of course, if your main computer is also your gaming/tv/entertainment rig, then you really do need to take that into account.
As an aside, a 24" widescreen with two outboard 17" monitors in portrait mode gives you approximately the same DPI and vertical height on all 3 monitors, so you don't have the window shrinking/enlarging issues you have when you choose a multiple monitor setup with different size monitors. Setting the brightness and color on such a setup correctly, though, is annoying, but not as annoying as having mismatched monitors.
Keep in mind that many people don't like dual monitors because their first extended experience with them was in the wrong setup. Don't be afraid to try out a few different setups.