The standard for DVD is 5.1 which is exactly what THX uses.  The 6 channels can be confusing though.  There's actually 5 channels (left, center, right front, and left and right surround) and a LFE (low frequency effects channel).  In film, the LFE is used specifically for low freq. sound effects such as explosions, earthquakes, etc.  In music, there is a bass management circuit during playback that takes the low freq energy
from the 5 main channels and folds it into the LFE. 

5.1 multichannel was invented for movies. In a movie, you often have a moving sound source (the archetypical plane flying by); surround sound would allegedly add to the realism of the experience, increasing the thrill of going to a movie theater (in an era where movie theaters are consistently losing market to home video). So far so good. The problem is, the quest for realism misses the point altogether of what movies are about.

Does Star Wars Episode have more realistic sound than older, non-surround sound movies? Sure, but that is not the point! We all are willing to relinquish some measure of realism when we go to the movies: watching a flat screen, taking giant leaps in narrative space and time, following camera movements, these are all "unrealistic" in a sense; in
effect, they are part of the cinematic language, a language we all are so used to that we don't even think about it. But in the end, it is the language that matters; if surround sound effects ADD meaning to the cinematic language, they are a valuable improvement; otherwise, they are only cheap thrills: effective, showy, but ultimately superficial.

My contention is that so far, the latter is the case.  I would contrast the introduction of surround sound with the original addition of sound and color to movies, which changed the cinematic language forever, especially in the hands of great directors (witness Kieslowski's Three Colors, Bergman's Cries and Whispers, Kubrick's 2001, Coppola's
Godfather(s) and Rumble Fish). I would dare anyone to come up with a movie where surround sound plays such an important role as the color palette does in The Godfather, or the soundtrack (plain, old stereo) in 2001 or Barry Lindon.