The DesignTechnica gang has an editorial up about the importance of each individual speaker in a 5.1 setup; more specifically, the importance of the center channel. If you're a surround sound n00b, & you've just bought your first set of fancy-schmancy speakers, you should prolly head on over & give this article some attention...

"Why is the center so crucial? Doesn’t that seem counterintuitive? After all, stereo systems get along without any center channel at all. However, movie mixes depend heavily on the center channel to deliver both dialogue and the leading edge of those action-movie surround effects that make our hearts beat faster."