I concur with AVRat. If you can, build it bigger than you need to.
I bought a wall unit entertainment center about 10 years ago that was a very nice dark oak with carvings, about 12 feet wide total. Our 36" CRT tv fit in there perfectly. We no longer have the CRT, we moved to a house a with builtin cabinets, and have no tv's that fit that unit anymore, so it's been sitting in the garage unused for 4 years and unable to sell it. Not the same situation as yours of course, but in retrospect it would have been nice to have gone with a unit that was more flexible in the size of the tv it could hold...it's hard to imagine ever being unhappy with a 50" plasma, but someday you might want to upgrade to something bigger, plan ahead if you can.
