Listening to music while driving is one of the most prolific shared driving habits that there is, and this reflects on people's increasing awareness of and sensitivity to the quality of the sound. Many people simply won't put up with inferior sound, and after you've heard a tripped out, highly-tweaked system there's just no going back.
So, buying new speakers is a great way to significantly ramp up the sound quality you experience in your vehicle. This website goes over the key points to choosing the best car speakers. Most car speakers fall into one of two categories: component speakers and full-range speakers. For the least amount of effort you'll want to go for full-range.