That depends on what you do with your system more -- watch movies or listen to music. While I think a sub will make the most difference regardless of your habits, if you watch a lot more movies than listen to music, I think you should get the center. Anchoring that dialog to your screen will make the voices much clearer. An it's always nice to hear the voices.
Then again, if you watch a lot of movies that would benefit from a subwoofer, then getting that first makes the most sense.
My vote? Get a sub.
