Once you've used a multi-display rig, you'll never want to go back.
I run a triplehead setup at home, and a dualhead at work. I hate it when I have to work on a single monitor.
You can get an additional PCI card to run your second display, or you can get a new AGP card with dual, triple or quad heads.
Matrox makes the best multihead cards around, but they're not for those who like to count framerates (although my Parhelia plays every game I've tried on it with acceptable rates).