I know when I purchased my Mac Pro, the default video card was available to be purchased multiple times... such that if you installed two, you had four monitor ports. I was told by the guy from Apple that installing it was just as simple and intuitive as configuring the typical side-by-side display.
In fact, just today I was noticing a couple of abandoned 19" LCD's in the corner of the office... wondering how I could get four monitors running on my G5, and how much it would dog the system down.