The other thing to consider is how big of a screen you are going to get, where you are going to mount the projector at, and how bright it will be. If you are building this room, is it going to be a basement or a windowless room? Have you thought about where you will place all of the receivers, DVD players, etc... at? Are they going to be in the wall with proper air flow, in a closet, or behind or to the side of the couch. Remember that they put off light and it should be controlled.
Please build in cable runs behind the walls.
Also, I would recommend either getting speakers in the wall or pre-running all of the audio cable and having face plates installed around the room just like an electrical outlet. (Just remember that audio cable should be a fair distance away from the power cables, but you probably knew that.)