Most mounts have four mounting points for a reason - to adequately support the weight of your beautiful new plasma TV. I advise using all of them. Remember - the warranty doesn't cover damage due to the set falling off the wall.
Jim is certainly right about the sound system, the room is too small to effectively use a 7.1 system. So stick with your 5.1 & put the savings toward a higher end projector. I am very curious, however, why mounting on a shelf or from the ceiling can determine whether you need an LCD or a DLP.