Being able to choose the right components of your system all depend on what end product you are looking for in your car stereo system. A big part of this choice is what music you typically listen to. For example I give some examples and typically what kind of sounds you want to look for to optimize your music.
Hip-Hop/R&B- When listening to these types of music it is quite obvious you are looking for bass in your system, but not just one type of bass. You are looking for good range in your bass, from the lowest lows all the way up to your mid bass that you feel in your chest. For this type of build you want big/multiple subs and lots of power along with good speakers that will give you good midbass with the possibility of adding in more speakers than you are typically used to.
Country- Believe it or not but country does have quite the amount of bass in the songs but there is a big range of sounds not found in the Hip-Hop/R&B category. Here you could get away with a middle to low end sub here with the typical four speaker set up, but you will want to upgrade your speakers and get a mild amp on them so that you can listen to the wide range of sounds country presents.
Rock/Heavy Metal- This music can sometimes be fast paced so you want some a system that can respond in good time so that your sound does not sound distorted. There is not a whole lot of low end bass typically but with some emerging rock/heavy metal it is getting mixed in. Most of the drums fall into the mid bass area and you would want to go with a smaller sub that will hit the mid range better and has faster response time. You will also want a good component speaker set up that will enhance your mid bass and the range of sounds you find in metal.
Pop- this genre falls into and melds with Hip/Hop ranges of sound but the singers in this genre are usually female or have a higher pitched voice. You will want a good range of bass but not too much that it will overpower the words being sang because in this genre it is often more important than the beat. One medium grade sub should be plenty here and a good set of components will allow the music to be 'felt' and the words to be heard.
That is all I can think of for popular genres for now, if anyone has any that they would like to be added please let me know.