I want to cover top half of my brick fireplace with drywall, and install a 47" flatscreen TV. How can I attach the 2x4's to the brick to support the drywall and weight of TV (approx. 75 lbs). Lag screws? What's best? Thank you!

Screws and anchors. Lag screws alone won't work. Predrill 2x4, hold in place, pass masonry bit through 2x4 and mark place to drill brick with masonry bit. Remove 2x4, drill hole, insert either plastic or lead anchor. Reinstall 2x4 and screw to brick.

I certainly go along with drilling for anchors but would also add some construction adhesive to the back side of the lumber. 2x4 will work but you do not have to go that large. Whatever you pick remember to countersink the heads of the anchors. The TV mounting plate will have a specific bolting pttern so make sure you match that pattern with your lumber. I would follow Harolds advice on how to locate the correct location to drill and would get anchors to match the bolt pattern of the mount. You do not want to mount the 2x4 and then mount the TV to the 2x4 alone, get it into the brick too.