You could start off with replacing the front door speakers with a quality component set. Then work from there.

 

Start out with replacing the head unit, something like a good Alpine, Pioneer, or Kenwood works good. If you have the subaru "upgrade" 6-1/2" front components, those are pretty decent speakers, but they need a good crossover instead of the factory cap on the tweeter. it is worthwhile to invest in a good set of component front speakers though, just make sure they are sized properly and don't hit the door panel at hard bass. The rear speakers are just sound filler, and the stock ones work fine.

My system is simply a Alpine 9813 head unit and Focal Polyglass components in the front doors. It sounds incredible compared to the stock system. I plan on investing in a good amp and some coaxial rear speakers in the near future though...