Allow me to add that when we built ours we intended it to be the center of our home entertainment area - so we put a 32" widescreen TV in ours - had to totally modify the plans but it worked!
As far as weight, I'll only echo what AtomSmasher said, except that the bigger you go the bigger the brace obviously....because our TV was going to be SO huge, I used 2 x 4's, instead of 2x2's. Also used the screw in 18" long metal "L braces" (4 of them). Our cabinet has been in our house, fully functional for 2 years, and that shelf has not budged. Very sturdy. We did, however, also mount ours horizontal to make TV viewing easier....but need to raise the unit up a little so that the screen is more in the center of the bezel.
I can hear some of you laughing now as you try to figure that out - I can't remember the exact name of the brace at 3:40 AM!!