Monitors give you headaches because of refresh rate (does not equal frames per second), and if your eyes are too close, tiring your close-vision muscles.
At that resolution I'm not sure, but one will definately be more than enough for 1280x1024, and probably enough for 1600xwahtever. You don't HAVE to always run at max resolution, by the way
I'm not too sure about how these duel cards work, but perhaps it would be possible for you to just buy one, see what it's like, and then buy another one if you feel it is required?