There are a few companies that offer full upgrade packages. One even offers a package deal with a subwoofer/amp setup.
This guy did a nice install:
http://www.m3post.com/forums/showthread.php?t=183025
These guys have the sub upgrade packages:
http://www.musicarnw.com/
Thread with pix of what can be done:
http://www.m3post.com/forums/showthr...woofer+upgrade
But the best thing you can do at this moment is search. There is tons of info on the site relating to this. Whether you want to upgrade the under seat subs, put one in the trunk, swap out all the speakers, or do a full blown setup, it is all here somewhere you just have to find it. I have spent hours reading on all of it. Just look around for a bit, you wont be disappointed...