car audio is a strange one, and one that people constantly do wrong. there are 4 main things to consider in car audio. quality speakers, quality amplification, quality source, and quality acoustic treatment, and its that last one people always ignore or forget, but its the most important.
car audio is a crash course in diminishing returns. if you just replace the speakers, anything over $150 is pretty much wasted money. above that you will get a different sound, but not much improvement. add a subwoofer and you will get the low end response production cars struggle with, but will sound detached from the rest of the sound ie you will hear the music played by the spears and the music played by the subwoofer, not one nice even sound.
if you replace the head unit (the thing with the CD player and volume dial on it) and the speakers, you get another improvement, generally in clarity and dynamic range. more money = more features and slight increase in clarity.
if you add an amplifier, you get clearer sound, more dynamic range, and more volume (if thats your thing). more money, better sound, but the returns diminishes rapidly.
add acoustic dampening and you will get a whole new level of performance. your $150 speakers will sound better than $1000 speakers in a non treated car, clarity will increase no end, and your driving pleasure increases hugely because it feels like a luxury car, no matter what heap of junk you drive. the music will no longer be playing over the road noise, it will have replaced it. details previously lost in the background noise will become audible and enjoyable.
with the acoustics of a car. just like home audio, you can have the best speakers in the world but if your speaker enclosures are like shoeboxes or tin cans they will sound terrible. so dynamat/similar should be used and all sheet metal areas (floors, doors, boot/trunk lid, roof, firewall, footwells) then over that some "decoupling foam" then a layer of mass loaded vinyl (MLV) to dampen exterior noises, vibrations, rattles, and most important resonance. the dynamat adds mass to reduce the resonance of the metal panels, the mass loaded vinyl absorbs sound waves and the decoupling foam prevents transmission of vibration between the layers, absorbing the energy. also sandwiching some dynamat and foam between the metal body panels the speakers will decouple the speakers from the metal panels.
here is probably the most concise explanation of what the three main components of automotive sound deadening do, and also how to keep the costs down. (he calls dynamat by its proper name, constrained layer dampening or CLD)
if you have never been in a properly sound deadened car you will probably not understand why it is worth the huge expense, but seriously, a well sound deadened car - no matter what car it is - Will instantly feel like a Bentley. once a car is deadened the sky is your limit to how good you want the sound to become. suddenly the rules of diminishing returns have gone from $150 to $1,500. any improvement is easy to hear and appreciate and you will really enjoy driving a quiet car.
generally for top quality you want to sound deaden, change head unit for a pioneer/kenwood/alpine with 2/3 RCA pre-outs (so you can add 3 amplifers, one for the front speakers, subwoofer and rear speakers (optional)) add a couple of amplifiers (1 2/4 channel amp for the speakers, plus one mono block per subwoofer).
then change the speakers for something by JL, Focal, DLS, Diamond, Hertz, or Rainbow preferably 3 way component speakers with individual tweeters, midrange and a mid-bass speakers. top end audio installs generally install the midrange and tweeters into the A pillers (windscreen pillars) and/or the dashboard to give a frontal sound stage. the midbass then go in the sides of the footwells or the doors, preferably angled towards the listener.
a good install usually uses fibreglass to create custom enclosures for the speakers in the pillars, dash and footwells, then again for the subwoofers in the boot.
so, a basic, low effort and good value for money install would be to add dynamat to the doors to reduce resonance (dynamat does NOT block outside noise very well, this is what foam and MLV do), replace the stock speakers with 6.5" 2-way component speakers (tweeter and midrange) made by alpine (not the alpine type R speakers, they are terrible), DLS, Diamond, rainbow, hertz or focal, and replace the head unit with a decent alpine, kenwood or pioneer. keep an eye on sensitivity ratings of speakers as some NEED an amplifier to sound decent, other ones just sound better amped.
then from there you can add a subwoofer (or two!), amplifers, more sound deadening (floor, trunk, firewall, roof) all to whatever budget you have.
generally 10" subs give a fast punchy sound, 12" subs a little slower. ported subwoofers boxes are more efficient than sealed boxes and can produce lower frequencies, but again are slower and less punchy. using 2 subwoofers make them sound more effortless and a cleaner sound, but you lose the use of your boot/trunk.
avoid no-brand amplifers, and any amplifer that quotes 1000watts power for a cheap price, they lie! use RMS power quotes NOT the max power quotes, and look at THD values (no more than 1% THD. in car audio THD values are inherently higher than home hifi so less than 0.1% THD are quite rare in anything but top end stuff)
hope some of this was useful or interesting.