bigjohn has a point. Each component of HT is really important in it's own way. Its only when you get into multiple surround speakers does it become a matter of taste or want. But you pretty much need the basic 5.1 system to get everything out of your home theater.
The subwoofer will add an extra sense of presense and umph to the sound. Don't skimp out here. Trust me. A good sub goes a long way.
The surrounds will get you that "something was behind, now it's in front" kind of experience. It's definitly something you'll want down the road.
And of course the center is designed to carry the bulk of the dialogue.
Personally at this point in time, I'd go with the subwoofer. It the most versatile piece you could add to really enhance both movies and music. And as odd as it may sound, I'd then go for the center, and then do surrounds last. Or try to save and do the center and surrounds at the same time.
Good luck!
