Depends hugely on what you plan to do, and how much money you can afford. In general, I'd say wide-screen flat-panel (typically LCD) is the way to go because they will accommodate both older styles (4:3 proportion) and newer wide-screen (16:9 proportion) displays. Actually, most of these for computer use are 16:10 proportions, like a screen with 1680 x 1050 pixels, to allow for a Taskbar at the bottom leaving the 1680 x 945 area above for true 16:9 display.
My own system (a couple years old) has a "20.9" Wide-Screen LCD display. It has about the same height as a 19" old-style (4:3 ratio) screen, but with extra width for the wider proportion displays. The extra space is nice for laying out several windows, or for wide spreadsheets. And it works just right for playing DVD's. The one I have at work is even bigger and really nice, but I'm not sure I would spend the money to buy one for home.
Be aware that LCD densities are pretty similar. That is, most of these have almost the same number of pixels per inch. So many 20" to 22"wide-screens are 1680 x 1050 pixels, whereas a larger 24" one may have 1920 x 1200 pixels. More pixels means more $$, so the bigger you go the more pricey! You also pay more for higher brightness (cd/m2, which is candellas per square meter), for faster response times (in ms or millliseconds) and for higher Contrast Ratio (bright white vs. black), Fast response times are important for fast-moving items such as gaming, but not really for desktop computing like docs and spreadsheets. And, of course, you pay more for reliability and manufacturer reputation.
If you opt for widescreen, you need to make sure your video output (either on the mobo or as a add-in PCI card) can provide the matching signal. For example, a large widescreen with high pixel count is wasted on a cheap video card that can't give it such a high-res signal. This is especially important with LCD displays because they have a fixed pixel count, and your best display will be from a card that exactly matches the LCD monitor - say, feeding a 1680 x 1050 image into a 21" LCD with 1680 x 1050 pixels.
I highly recommend searching around for a bargain deal on a good unit. We bought one for my daughter this past Christmas as part of a whole new system we built. In the course of searching many on-line sites for components, we stumbled across one shop that had a great deal on a video card and also had a sale on a good monitor at prices similar to much smaller units. But other sites did not have that particular deal available. And although Tom's does not endorse any stores, I'm like a lot of people, happy with the selection, prices and service from Newegg, so include them in your search.
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