I am certainly not an expert on this, but from all the reading I've done, SLI and Crossfire are meant to use multiple GPUs to improve performance on a single display. That's not what I want to do.
I want to have multiple displays. I have 2 monitors right now and want to add a third. As far as I can tell, everyone agrees that to do that you simply add a second video card. There are warnings about driver incompatibilities and whatnot, but I haven't even gotten to the operating system yet.