Any GPU today will handle two monitors, so long as it has two plugs (DVI+VGA, DVI+VGA, HDMI+DVI, you get the idea). To run three or more screens, though, you need a second card- not running in SLI (nVidia) or Crossfire (ATI).
What's more, just about any modern motherboard with integrated video will let you keep its outputs enabled alongside a plug-in card, so long as the drivers are compatible. For example, my AMD 780G motherboard can run two monitors, and I can run two more if I plug in a Radeon- even something low-end.
If you are running Vista, you need to keep the same type of card. So if you have an ATI one, buy another (same goes for nVidia). With XP, it really doesn't matter.
I'd like to point out that for Vista, you do want something running on PCI-E. I got myself a PCI bus GeForce 6200 128MB (since I have only one PCI-E x16 slot) to run a third monitor and it's slow and causes problems. Moving windows is noticeably laggy, not even Flash video runs properly on that screen and when trying to run OpenGL games even on the primary graphics card (8800GT) it has non-existant framerate unless you disable the 3rd monitor.