My advise to you is don't hang either above your fireplace. I know it seems like a good idea to free up space, but in the end you will regret it. First of all, the heat coming up from the fireplace, even if it is just a gas one is very bad for both LCD and plasma. Second, the TV will be much too high on the wall to watch it comfortably. It will literally be a pain in the neck. I recently bought and moved into a new house and on the only wall in the family room where the TV could go, the builder put the fireplace in the center of the wall, and then put in two built in units with a solid shelf in each one that was mantle height on each side of the fireplace, that made it so that I has no choice but to put my TV on the shelf. To make a long story short I have now ripped out that shelf and am using a TV stand the same width as the TV inside the built in. Placing the TV above the fireplace will look very nice when you are NOT watching TV, but you will hate it when you are watching TV. Buy a TV stand that will be the same width as your TV and keep your TV "eye level". That way you wont be taking up a lot of space with an entertainment center and you will still have a place to put your DVD player Sat/Cable box etc. Did you forget about them? They wont fit above the mantle, you'll still need a place to put them. In short, DON'T DO IT!
I'm looking to buy a flat panel tv but I have a small den so I would like to make more room by centering the tv over my gas fireplace and getting rid of my oversize entertainment center. I was looking for a 42-46 inch flat panel that would mirror the opening of my fireplace with the mantel sepperating them. Which one will handle the occassional lighting during the winter best LCD or Plasma? The fire is never on longer then an hour.