If you can swing the $$, crossfire would be a good thing to do to avoid lowering the resolutions on games. As far as sizes, I would try and stick with the same size if you can, It may look odd to have a 27" in the center, and like 23 or 24 on the sides. You also need to be sure you use DisplayPort on at least one of the monitors, whether it's built in, or you get an Active adapter separately.
I personally have a 24.6 as my main one, and had two 23's for my side ones, that looked OK for my taste, I guess it just depends on how picky you want to be. Stick with monitors of the same res, though.