I'm running 4 monitors on one machine (Win 7 x64 Ultimate) and my desktop spans all 4 monitors.  My video card manages the first two, and then I have two of these USB 3.0 monitor adapters that I connect the other two to.  Works brilliantly, and flawlessly for me and I don't need another video card:

http://www.newegg.com/Pro...x?Item=N82E16815158325

The Win 7 OS default monitor setup app/settings window allows me to manage all 4 monitors seamlessly even deciding which one is Number 1, Number 2, etc, etc, and also how to access each next monitor from the current mouse position (move left, right, or top, bottom).

PS: in my thumbnail photo (member icon), the 4th monitor is being hidden by the red chair in the foreground because that monitor is set low, and built into my desk.