What's the best way to Fill the space above a fireplace that was made for 34" tube tv?

My living room has a space that was built in above the fireplace that was perfect for holding our old 34" tv. We’ve recently got a 50" plasma and I want to mount it on the wall in the same spot. Does anyone have any suggestions as to the best way to fill this spot? Studs? Plaster? I need to make it sturdy enough to hold the mounting bracket and TV.