IN CAR ENTERTAINMENT (ICE)

ICE is a personal choice, and what you go for is going to be determined by what you want, how much you are willing to spend, and how much room you are willing to sacrifice. The general steps of upgrade, would be front speakers, head-unit (source), rear speakers, amps & subs.
Now, before you start to think about what you want to buy, you have to think about what you want out of your system. Some people want the most volume they can get, others the highest quality but most people just want a CD player and a good quality sound.

Head Unit or CD-Changer Upgrades are one of the first areas that should be given attention as the standard equipment isn't going to produce a very high quality output.

Speaker Upgrades are another primary area of attention and, even with the standard head unit, will produce a noticable improvement in sound quality.

Adding External Amps to power the speakers is when the volume and quality really take a step to the next level.

Adding Subwoofers will release a whole new area of frequencies into the sound - low frequencies (BASS!)