There is a ton of information on this site about upgrading the audio.
Follow this thread.
http://986forum.com/forums/showthread.php?t=15893
I put in the double din, Avic D3 with DVD and Navigation. Head unit, install kit with instructions, new subs and new amp. I did it all for around $1,000 and it's been the best darn upgrade I made to the Boxster especially when I plug in a music video DVD.
Good luck,
-Steve