I would recommend running a four channel amplifier using component speakers up front and co-axial speakers in the rear. You can add more speakers up front, but it depends on what you're trying to accomplish in your car. Do you want it louder? Do you want it clearer? Any system will benefit from an amplifier. More power to your speakers will give you better sound because you'll have more accurate control over the speaker cone. This extra power will result in better clarity, not only at high volume, but low volume too.