If one monitor is good, two is better, right?
I've been using two monitors simultaneously with my laptop for over 3 years now. Let me tell ya, it's great having all that desktop real estate to play with. Your mouse moves from screen to screen and you can move open windows from one screen to the other just as if you were using a single large monitor.
So how do you set it up?
Basically, all you need to have is two video adapters (cards). Your computer already has one, but you'll need to either install another one yourself or take your computer to someone who can.
Keep in mind that depending on your configuration, you may not have any space (or interrupts) left for adding another video card. Due to the millions of possible configurations, I don't really have any guidelines to give you in regard to this. If you're not sure, take your machine into a computer shop and have them look at it.
If you're using a laptop, you may already have multiple video adapters. One for your LCD display and one for a standard monitor. Just plug your regular monitor in and go. It's that easy. Of course, you'll want to confirm this before you go out and buy a new monitor, buy most newer laptops will support this configuration.
Once you get the hardware taken care of, the rest is easy. Right-click your desktop and select "Properties" from the resulting menu. Hit the "Settings" tab.
You'll see both monitors displayed. You can set resolution and color depth for each individually as well as drag the monitor icons themselves into their real world positions (makes transitions from one monitor to the next smoother).