If you want the best go with DynAudio or Focal Utopia's, but be prepared to spend $$$. Next step down would be your MbQuarts and so on. If you an buy the speakers @ bestbuy or circuit city they won't be very good(IMHO). You may try Crutchfield as they can help out with wiring harness's.
I'd reccomened replacing the head unit because the sound is only a good as the source! :ah: Then I'd use a different AMP and speakers.....no big deal just $$$.
Check this out for a good idea.
http://www.carltonbale.com/boxster/audio/index.html