So you want more of a sound quality system. Well your going to want to spend most of your money on a really good set of component speakers for the front, and a nice amp to power them. You are also going to want to buy some sound deadener for the doors and the trunk. ex( http://www.sounddeadenershowdown.com/cgi-bin/index.cgi )
An image dynamics sub would probably be the best for you, they have really good sound quality in a sealed box and can get loud if needed.
http://www.woofersetc.com/p5806/IDQ10V3D2--Image-Dynamics-10"-IDQ-Version-3-Subwoofer.htm
Just one of those in a sealed box will sound amazing
Rainbow and morel makes some amazing speakers, but they are really pricey.
These ones are supposed to be really good for the price http://www.woofersetc.com/p6952/RSD65cs--Phoenix-Gold-65"-Component-Speaker-System.htm