The easiest approach (especially if you're running Windows 7) is to change your display card to one that has dual monitor outputs.
You may also be able to simply add another display card to your computer, but if you're using the display hardware on the motherboard you may have to disable it before the computer will look at the new card (which leaves you with still a single display)
On at least two of my MoBos, I've been able to use both the onboard video and a video card to have dual monitors working. Using Win XP or Win 7.
That being said, most video cards have dual monitor outputs anyway, so to improve system performance, I would disable onboard video and use both outputs from the video card.