Stereo install

How much you have to spend makes a big difference in what you install.

A decent set of 6x9s in proper boxes can create good overall range for the "normal" consumer. The big key is to buy an amp. Head units (even good ones) only put out a limited amount of power. Turn them up too high and they start to "clip". Using pre-amps to a seperate amp, even one of moderate power, will usually give you much better, cleaner sound.

The only way to get true low audio is to go with a subwoofer.

While I currently have a good amp (Kenwood) and sub (JL Audio) my head unit is a cheap Panasonic POS ($70 on clearance) and a decent set of Pioneer 6x9s. Nothing spectacular, but they were better than what I had (nothing). I plan on doing a full custom dash on the car, and when I do I am going to install a good head unit with USB, and stash a USB drive in the glove box. Currently my amp is fed only by A&B channels, since the cheap head didn't have a dedicated sub pre-amp...as a result, I think there is some quality I am missing out on. That will also get fixed on the new head unit.

As dingo mentioned, the sub airspace is what's critical..the 6x9s...not so much, but just having them backed by the trunk will sound like ***....ask me how I know THAT one.