They may conjure up the image of frantic day traders, but few pieces of technology can make your work at a desk more serene than a second monitor.

Forget about constantly minimizing and maximizing windows to read a new email (which usually is spam, anyway). Put Outlook on its own monitor, and go about your work in the other, calmly looking to your side whenever you want to check your mail. Or the calendar, or a spreadsheet, or your favorite sports Web site.

When given extra screen real estate, most people have no trouble putting it to good use. And your mouse moves effortlessly between them as though they were one big space.

Fortunately, for both Windows and Mac users, adding that extra monitor -- or two -- is one of the easiest big improvements you can make to your PC. (While most people are happy with two screens, Microsoft researchers found that the optimal desktop setup involves three: a monitor in the middle of your field of vision and then one on each side.)

All you need are the extra monitors, and extra spaces on your PC to plug them in. Look in back, where your current monitor is plugged in, to see if you have the outlet for another. If you're unsure about what port is for what peripheral, check your machine's documentation.

There is a good chance your computer hardware already supports a second monitor. But if it doesn't, you need to buy an extra video card, sometimes called a graphics card, and stick it into an empty slot inside your computer. Companies like ATI and nVidia sell the cards, priced from $40 or so on up. But unless you're a gamer, you don't need the more expensive cards; just make sure the one you buy has two outputs, one for each monitor. Most do. (For instructions on installing a video card, once again, check your machine's documentation.)

A quick word on video outputs, or the actual plugs where you attach your monitor cable. They come in two varieties: the older sort, VGA, which are about half an inch wide and have 15 small round holes stacked in three rows; and DVI, which are longer and have a more complicated pattern of small square and rectangular holes.

DVI outputs produce a digital image that is sharper than the analog VGA variety; if possible, get both monitors and video cards that have DVI on all outputs. Note that you can take the signal from a DVI port to a VGA-equipped monitor via a special adapter.

If you are running any modern version of either Windows or the Macintosh operating system, once you have plugged in any extra video card and the new monitor(s), your computer will know about the new displays as soon as you boot up. After a little bit of configuration work, you will be on your way.

In Windows, go to the Display section of the Control Panel. In the tab for Settings, you will see two or more monitors, depending on the number connected to the PC. One is the primary monitor, one the secondary; click the Identify button and you will see which is which.

Highlight the secondary monitor, and select the box that says, "Extend my Windows desktop onto this monitor." And make sure to drag around the icons for the monitors so they are lined up on the screen the same way as they are on your desk. Otherwise, your mouse won't behave the way you want it to.

When you're done, your desktop will extend into the new monitor, as if your screen had gotten larger. You can, for instance, drag the window for an entire application onto the new screen.

Now, as you move the mouse off the edge of the first screen, you should see the cursor scroll onto the second. You're in business!

The process for configuring a second display on a Mac is roughly the same as on a Windows machine, and shouldn't be difficult even for a beginning user. Desktop Macs such as the PowerMac and iMac support two monitors when shipped; most of Apple's notebook computers have an outlet for an added monitor, too. By and large, only the small, self-contained Apple PCs like the Mac Mini don't have the ability to support a second display.

Most office-oriented applications behave fine with multiple monitors. There can be occasional annoyances at first; for example, a dialog box for a program you're using on one monitor might open up on the other monitor. Both ATI and nVidia say that the utility programs they include with their cards will tame those instances of unruliness.

A good source of troubleshooting information about multiple monitors -- gamers seem to be always reporting problems -- is the Multi-Monitor Resources Web site at RealTimeSoft.com/multimon. Besides tutorials on the basics of multimonitors, there are forums where users can look for help. The site also sells UltraMon, a $40 multimonitor utility that is similar to the software provided by the display-card makers.

How big should a second monitor be? As big as possible, of course. Both Dell and Apple sell 30-inch displays that make you feel like you're sitting in a theater. The price: $2,100 to $2,400. If that's a bit rich for your blood, you can find 17-inch LCDs starting in the $160 range, while 20-inch LCDs usually begin at roughly twice as much.

And while most people by now prefer LCDs, don't overlook traditional CRTs; they are hard to beat when watching video. You should also think about monitors that can be flipped between portrait and landscape mode; most monitors are horizontal, while Web sites and documents tend to be vertical.

I have a $400 20-inch Samsung that I use in the portrait mode, and it's a treat to be able to see all of a Web page at once. Think of how much more productive I am on YouTube!