If you start typing "eyefinity" into a Google search, one of the suggestions is "eyefinity setup." Unless someone here has already done a 6-screen setup, that's what people will be referencing. The thing you'll probably need is a second graphics card for more video ports. You shouldn't necessarily need a matched GPU to your current, but it wouldn't hurt.
You should also keep your posts condensed, but Crossfire is AMD's way of driving more power into a single screen by using 2 GPUs (it could be more screens, but the general idea is more GPUs for one image). You would be link two matched GPUs over a proprietary bridge, and then probably take a look at AMD guides to work out the kinks in the setup. I wouldn't recommend that, though. A single, fast GPU is generally higher value than throwing in more cards.
I could be a little off on the needs for more than 3 displays, but that is how a friend of mine hooked up 4 displays (2 4870's, I think).