Check this out
http://www.knudsons.com/images/MyDesk.jpg
Although as Ben mention, most games won't take advantage of them, hopefully sometime in the future.
First, I would ADD a dual card. even gamers ones aren't expensive to (Not counting PCIe).
If you add more than one card, stick with the same chip manufacturer, Nvidia, ATI ....
One final word, once you start running 2, you will NEVER go back. Having massive amounts of screen real estate is addictive. (I'm hoping for a Second Quad setup next to this one, and maybe even add 2 more still to this one)