Plasma
or Projector?
For maximum image size you should use a projector, these are much
cheaper than large (50"+) plasma screens.

However projectors require the room to be dark, where this is not possible then
a plasma screen is a better option.
How
many speakers?
A basic system consists of five speakers and a subwoofer (5.1). More
elaborate systems use six, seven or more speakers plus subwoofers.