Answers to a few of your questions
Terinox, the size of speaker does matter unless you plan on doing modification. Your car (Saturn, right?) probably has a 6.5 S size (fancy way of saying 6 3/4") As far as what to buy it makes the most sense to go out and listen to whatever speakers fit your car (the people at the store should know) and you budget and buy those. Typically the best sounding speakers should be placed in the front of the vehicle. How often do you drive from the trunk or backseat? I can't understand why so many people put better speakers in the back. You don't go to a concert and turn your back to the stage do you? Also a component set is a good option but unless you are going to invest in a kickpanel setup you may as well just replace the factory door speakers. I suggest Boston Acoustics, MB Quart, Eclipse, Rockford Fosgate, Focal. When it's all said and done it's YOUR car so buy what you want. If I can help with anything ranging from install tips to actual sound qualities lemme know. Mark