There's a thread in here somewhere that discusses AH and widescreen gaming.
Basically, because of how AH does views, if you have a widescreen monitor you don't get any more usable view area. What you get instead is the same horizontal viewing angle and the top and bottom of the view is cut off.
In fact, you'd be better off from a gameplay point of view getting a widescreen monitor and rotating it into portrait mode. You'd keep your 90 deg horizontal view angle, but get a whole lot more vertical view angle, which is far more important in the game. It would probably look terrible, but you'd have a heck of a view advantage over using a widescreen monitor in it's normal orientation.
So... For AH, at least for now, a regular 4:3 aspect ratio ("square") monitor is going to look better, because AH doesn't grok widescreen monitors. Yet.