From what I have experienced, center speaker was usually for the verbal aspect of the game, the music and sound effects make use of the other 4 speakers plus the sub.

Its cool what you can do. I was playing with Assassins creed and depending on where your character is facing, voices will come from different speakers. Realistic!

A surround system is awesome for gameplaying. Alot of gamers miss this aspect and worry about a big tv or hdtv. Having both makes the expierence greater. Anyways using a 5.1 setup will use all the speakers. The center will being for talking and any sound effects that deal with in front of you. The right and the left front speakers are used for the music and and sound effects that deal with the front sides. The rear speakers don't actually really do any music since there for sound effects behind you. There are settings on the receviers to adjust how loud each speaker is to customize it to your liking. Prologic II sounds good but dolby digital is the way to go if you have a ps3 or x360(also ps2 and xbox). There is a definte sound difference and these two consles need hooked up via hdmi or optical cable to get the dolby digital sound need to use 7.1 since its even more amazing then 5.1. Lair is the only game that I know that uses it so far though.

You always hear people getting a big TV and stopping there. Surround sound, if possible, is a must. You might spend up to $3,000 for a TV, maybe more and yet many people wouldn't spend a few hundred more for the sound? You can get a decent system for very cheap if you look around. And yes, it does make a difference. It goes without saying that it's great for movies too.

In something like Metroid Prime 3, would the level music be repeated in the rear speakers?

Is 16gauge wire good enough for rear speakers? The ones i have right now aren't long enough.

In resident Evil 4 it tells you when you have zombies behind you cause when they say something it comes from the rear speakers . Note that Wii doesn't have true 5.1 surround so probably doesn't come close to the experience on PS360 but it definitely improves the sound quality over that from 2 speakers.

This is the most important aspect that it adds.  It will make music scores sound much better but when you are able to place enemies because of their footsteps or conversations, you realise that you were at a definite disadvantage without surround sound.

I agree surround sound in gaming brings a whole new level of excitement. The Wii is meh, with only Pro-Logic sound. The PS3 and 360 are great offering Dolby Digital and DTS sound in-game. Playing games like Gears, MGS4, GTA4, and Halo 3 on these systems is awesome. A game like Silent Hill really offers a new experience as Konami uses the sound to create an even scarier atmosphere.