Yea you basically need a video card with dual output, the type doesn't really matter as you get adapters that go back and forth.
Another interesting option that I used to do before I got a new video card is
http://www.maxivista.com/ With this you can use any pc on the network's screen as an extension of your desktop, I use to run a 3 monitor setup, 2 of them running on one pc, and another running off a seperate pc, but linked using maxivista, it worked quite well, but ya I had CRT's back then and the space they took up on my desk just didn't make practical sense.
