Cheap 3.1 system - I've got a subwoofer and a pair of computer speakers hooked up to the plasma TV that is good enough for me. The subwoofer provides a full sound.
Low budget - you can get a 5.1 system (4 speakers and a subwoofer) on Craigslist for $100 on up. The idea is to place a speaker in each corner of the room. Movies and such recorded in a 5.1 format, you can hear footsteps moving behind you left to right, someone off to the right speaking, etc.
More budget - 7.1 systems (7 speakers, one in each corner, one front center, one left center, one right center, and a subwoofer) have a more refined sound.
A sound bar combines two speakers in one cabinet, handy if the room is too small for so many speakers.