Hi Gene,
To install rear speakers you will need the rear storage compartment.
here is what is entailed in doing this:
http://www.pelicanparts.com/techarticles/box_rear_speakers/box_rear_speakers.htm
You can get the speaker kit from this guy on ebay.
http://cgi.ebay.com/ebaymotors/Porsche-Boxster-Rear-Speaker-Kit-THE-Upgrade_W0QQitemZ320087582282QQihZ011QQcategoryZ42 612QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
There are a few posts on the forum regarding the GHL's sound, ultimately it looks like a matter of volume and low rpm resonance preferences, although I would definitely say considering the price of a GHL system (headers & mufflers), it probably isn't worth it performance-wise
If you are looking for inexpensive (as far as Porsche goes) exhaust that sounds good and will add some hp , check out this thread...
Performance Exhaust and Header for Boxster
specifically: RandallNeighbour's post #57.
Performance Exhaust and Header for Boxster
I hope this helps a little.
What part of Atlanta are you in? I am in Smyrna and work on Cobb Pkwy in Atlanta.