I have no personal experience of them, but Matrox do clever widgets that let you connect 2 or 3 monitors to a single-head graphics card.

http://www.matrox.com/graphics/en/products/gxm/

Alternatively (and again I speak from no experience at all) I believe you should be able to add a second graphics card to your PC (as you mention). I don't think it needs to be the same as your existing one, but I'm afraid I'm no expert here.