Prefer a video card setup that can drive 3 monitors over the Matrox things.
The Matrox thing will make 2 or more monitors appear as one. Under Linux you can hack xinerama to deal with this. The Mac support makes it annoying to use. I haven't tried with windows but I imagine its ok there.
But get a modern card that can drive 3 monitors. It should be eaiser.
I've been running 3 24" screens for years now. But am thinking of going to one 27.