the single biggest problem (other than the fact it's a cabrio

) are the crappy paper cone speakers Porsche puts in the vehicles. From there, it has to do w/ the size of the door speakers (tough to get low end from a 5.25" speaker) and then the sound stage (why we add rears). Once those issues are corrected, it becomes a power issue and thus some choose to upgrade the amp and/or head unit.
Having said that and not know exactly what your budget is or desired end results, you can vastly improve the sound for under $500 by adding rears, changing out the dash speakers to good coaxials, and making custom baffles to house 6.5" speakers where the doors currently have 5.25".
New head units that look stock - think along the lines of Nakamichi as they are black w/ orange lighting. Not sure if any are XM/Sirius ready though.
You can easily change the speakers yourself by following these 2 guides or search the archives for more answers.
http://boxster.wikia.com/images/b/b6/Boxster_speakers_update_article.pdf
http://boxster.wikia.com/images/b/bf/Door_Baffle_Template.pdf
Good luck!