Not sure what size speakers you have in your car but I recently added an audio system into my wife's car. For the front door speakers we went with these:
http://www.hertzaudiovideo.com/Doc/pdf_ecx165.pdfA local car audio place here in town is an authorized retailer for hertz audio. They cost me about $190 installed (for 2 speakers). I have them amped with an external amp pushing about 75 watts into each speaker and they sound pretty darn good for car speakers.
She has a sub added to the system so I am not sure how well these do on the low end by themselves.
MBquart was another car audio speaker that was highly recommended by my audio installer, but I never gave them a listen.
