Can you use more than one video card for just one monitor? I think i saw a video where a guy used like four to make crysis run well. Just wondering, thanks.
No. The cabling is internal using something called a "bridge" (SLi, CF). It connects two identical (nVidia) or same series (ATI) cards together to act as one/in sync with eachother.
Edit: It wouldn't make sense to use your method as you aren't increasing the total GPU power. Which is why SLi/CF makes more sense. Lots of people do it. It won't "double" your performance, but it'll sometimes get close.