If you are a 23yr old kid trying to save money - a muffler is not a good performance enhancing choice. The order of choice in PERFORMANCE would be 1-intake, 2-headers, 3-ecu.
If you are looking for SOUND enhancement, two low budget options are:
1-remove the 'snorkel', 2-remove some of the internal baffling from the muffler. Both of these are sweat only modifications, and don't cost anything unless you break something.
The snorkel removal, is taking off the tube between the air filter box and the grill on the drivers side. It enhances the sound by improving the engine snarl to your left ear. Pictures at
http://www.realtime.net/~rentner/Porsche/SideVents/SideVents.html
The Boxster Racing Board is a good resource for advice on performance modifications
http://www.iq.dynip.com/~racing/index.cgi/index.cgi?
price estimates: intake$400, headers $1000+, cat back exhaust $1000+, ECU $1000+ Together these give 30-40HP. Several mufflers ads claim 30HP, none of these claims are dyno proven and some exhausts are dyno proven to lose horsepower.
With a base Boxster, you can get meaningful horsepower improvement with a TPC supercharger or a 3.4L engine swap. With an S, the only meaningful improvement is a 3.4L engine swap.
If I were thinking of spending $400-$500 dollars on adding speed to my car, I would buy a helmet and go to the track with PCA. After 4 track days, including today at Lime Rock, my stock Boxster S is still way faster than I am as a driver.
P.S. If your plan is to add an intake, be very careful working with oiled filters. Oil in your intake can easily destroy the $150+MAF.