Hi aggafear, its always worth replacing the factory speakers.. usually low grade paper cone type that tend to dry out over the years, and can sound dull and dead. The frequency response is usually cut off at approx 15-18 khz... most after market speakers run to 20khz or 25 khz on really good ones. so you loose some of the high frequencies... plus the degraded sound as those paper cones get old. The factory ones are component sets... normally a tweeter on the dash and woofer in the doors.... they don't have a proper crossover network only a diode in the line to protect the tweeter... the after market ones ( the better ones ) have a crossover box that needs to be connected between the two. Thsi allows proper seperation of the frequencies... so you would need to hide that box inside the doors . i just bough a 2004 c5 and door speakers seem to be 16.5 cm ... take out the original and you should see its quite shallow... see how much depth you have with the window down and that will give you a max debth to work with. bearing in mind keeping the factory radio and your steering wheel controls.. you will need an effecient speaker 90db or higher as the factory radios don't put out much power. hope that helps