Put quite simply you need a graphics card that supports two monitors. If you are thinking of upgrading then also think about getting a card with TV-Out on it as well. Then you can see the actual TV image of your work while editing. Matrox Parhelia is a name that comes to mind.

In fact I'm having some thoughts on this myself. I don't have any references but can anyone confirm or deny the idea that if you have two monitors together you are much better off having flat panels as CRTs can 'interfere' with each other.