How about adding shelving and or storage to the entire fireplace wall? You could then put the TV even with your fireplace instead of over it. The TV isn't really the first thing you want people to notice, and having it too high can cause neck problems for viewers. You can plan on where to put all the associated electronics and electrical requirements at the same time. Otherwise, you will have a wonderful fireplace with a TV over it, a couch blocking it, and electronics wires all over the place.

PLEASE do not put a tv above the fireplace but if you must, then choose one of the cabinets designed to be decorative when the doors are closed hiding the big black rectangle you'll come to loathe.

Think function first. If your family watches a lot of TV/movies, mount your large screen over the mantel. We have two fireplaces, each with a screen over the mantel and no neck problems to report! Large screens mounted to the wall or sitting on a furniture piece always make the room look off balance. Be sure to have electrical wiring taken care of during the building process. Stacked stone would look great for finishing your fireplace. Use gray rather than brown tones. Post pictures of the finished room!