EdD

i think it really depends on your personal preference.

if you rarely watch movies, get a bookshelf speaker
if you watch a lot of movies, especially with a lot of rear effects, get something like the QS 4 or QS8
i have never heard a center channel being used as surrounds, except in a 6.1 setting where it was the center rear speaker. it was very nice watching Gladiator with it in the back. Although, remember, it is rare that you find a DVD as well done as Gladiator.