More screen space makes you more productive. It's a fact [PDF]. That's why I just upgraded my monitor to a 19-inch widescreen LCD display from Samsung (NewEgg). Well, the productivity thing, plus the fact that it was relatively cheap (under $200) and very snazzy looking.
Bigger is better, according to the experts. If you can't spring for an eight-foot wide one, you can still hook up multiple monitors to the same PC. Some computers can handle this pretty easily, for others, you'll need to buy a video card (eHow) or, even easier, an $89 kit from Tritton Technologies that allows you to add your second monitor through your USB port.