Good question. It really depends on what you want to add to the experience. The joy of having a good subwoofer should never be underestimated. On the flip side, having sound move behind you during a movie is a very cool experience.
Don't forget about adding a center channel, too. You certainly can run in "phantom" mode and have your front two speakers fake a center, but having a true center channel can be a good thing.
Adding rear speakers would probably be cheaper than adding a good sub, but a couple of questions:
1. What speakers do you have right now?
2. How big is your room?