As by request, and having no previous thread about this yet, I am making a thread here that will hopefully explain to some how to pick out a system for your car.
First off we all have similar troubles with choosing out our system, often based on two fields, sound quality, and sound pressure.

Quality and Loudness in Speakers

SQ stands for "sound quality," and can be applied to most any car audio equipment, from head units to speakers. But, how do we know how well a speaker sounds? SPL stands for sound pressure level, or how loud the speaker can play. The best way to tell both of these, is to hear for yourself, but not everyone can do this, therefore there are several factors we can look at to determine how good a speaker might sound.

Simply, we'll start off with what helps make speakers sound good and play loud.
Most often speakers are made with materials like polypropylene, or titanium coating, which are good, lightweight materials. In speaker design, companies try to lessen the weight of the speaker's cone as much as possible, which gives it a goodefficiency.
Efficiency is definedasper every1 watt you put into a speaker, you will hear acertain decibel level 1 meter away. Often 92+ is common for speakers, ranging from 85 - 102. Car speakers are much lessefficient than home speakers, though, and those areeven less efficient than soundstage speakers, but this isn't important right now. Therefore, companies try to keep their cones as light as possible to get better efficiency, because due to the lightness, the cones have more excursion and playability for volume. At the same time, companies need to consider that with alight cone, the speaker often becomes weak and/or frail, therefore they need toalso take into consideration how strong the cone is. Stock car speakers have great efficiencies because they are so light, but since their cones arepaper, they lack sound quality since the cones are very weak, and less solid, producing not as great of a sound.Manufacturers always take into consideration both these factors. If a speaker cone is strong and solid, it will probably weight more, therefore less efficient, but willsound better, and vice versa.

So when you choose your speakers, take these numbers into consideration. Thehigherthe efficiency, or sensitivity, the louder the speaker will be. Feel the cone for yourself, tap it lightly and see how strong it is, this will give you agood idea on both the loudness and quality.
(This is not stating that these are the only factors that contribute to a speakers sensitivity and quality, but they are incredibly important.)
There are also a few common misconceptions.
*Lets say you were to buy a companies line of speakers that come in almost all sizes. You will notice that the line of 6x9s will almost definitely have a better sensitivity than the 5.25s. Does this mean the cone material is more lightweight? No, since the speaker is larger, the increased displacement means the cone can push more air, therefore creating more volume.
*You also notice the subwoofer is a much larger speaker, yet has a worse efficiency than the regular speakers. Does this mean the last statement is false? No, remember that subwoofer cones are much, much heavier than regular car speakers. Due to their size, though as well as power you put through them, this is the reason they most often overpower regular speakers in loudness.

SQ

Speaker SQ
Speakers come in all sizes and shapes, most often 5.25, 6.5, 5x7 and 6x9. Often it is not easy to tell which type of speakers are good for your car, which is why several factors are important to take into consideration.
1.) Are you going to amplify
2.) Are you going to want tweeters
3.) What sound are you looking for

Coaxial speakers are the most commonly bought aftermarket speaker, and coaxial simply means the wire type, but is not oftenly credited as great quality, although they are most likely better than your stock speakers.
Regular non coaxial speakers are more likely to sound better, but not as commonly found.
Component speakers, are speakers that come with a crossover (the crossover redirects high frequency audio signals to a tweeter, and lower to the normal speaker) and tweeters. The tweeters play the highs and the speaker plays the mids and lows, this allows more dynamics in your audio, allowing you to properly place the tweeters according to what might sound best for you. Mathematical formulas will tell you the best placement of your speaker and tweeter for your hearing abilities based on the position of your head. These diagrams often come with component systems but not always.
If you are looking for a simple upgrade, coaxials are fine, if you think you want to continue down the audio line and not just replace, go for good speakers or components. Components, although, are best amplified, and not much sound difference will be heard in a component system and normal speakers without proper amplification.

Good companies to consider for audio solely depend on your taste in quality, which is why it's good for you to go to your local audio store and listen for as much as possible, but here are some commonly favorited speakers

Alpine
Phoenix Gold
dynaudio
boston
JL
infinity
and many more

Remember to consider
if you do not want to amplify
look for speakers with high sensitivity ratings
if you do plan to amplify, sensitivity will not be as important, but allows you a less sensitive but better sounding speaker to reach just as much loudness

Subwoofers

Hopefully you did not just skip down to this section before reading the speaker section, but in case you did, I will be referring a lot to terms I have defined in the speaker portion.
I will try to keep this portion simple and quick.

Subwoofers are the most commonly bought audio accessory in cars, yet few seem to truly understand several key important factors in subwoofers.

Subwoofers are often bought in pairs, can be wired multiple ways, and have much more dynamics than car speakers, so first we'll start off with wiring.

When it comes to wirign a subwoofer setup, you have to first consider your amplifier. (Subwoofers do not NEED to be amplified, but due to the weight of their cone and inefficiency, it's important to do so to hear it well).
Lets say your amplifier can take a minimum load of 1 ohm, you then want to consider if your subwoofer system can be wired down to this ohm load. (ohms are just resistance, and amplifiers need them to run, do not go below an amplifier's ohm rating or you will harm the amplifier). Lets say you just bought a sub, it is a single voice coil subwoofer at 2 ohms, this would be safe to hook up to your amplifier, since it's ohm rating is higher (more resistance is good). Your amplifier can take as many ohms as it wants, it just cannot drop BELOW it's suggested amount, too little resistance will harm it. The reason ohms are important is because less resistance means more loudness, while more resistance means less loudness.
Now lets say you purchased a sub with two voice coils at 4 ohms. This means you can wire the subwoofer several ways, to run 1 coil, both at parallel, or both as series. Parallel connections will halve the resistance as long as both coils (which will be the case) run at the same ohms, so parallel wiring a dual voice coil 4 ohm sub will halve it to 2, series wiring will add all ohms amount, since there's two 4 ohm resistances, it will become 8. If you leave it at just one coil (not suggested) it will stay at

 

Wow, very nice write up.

 

Word. Maybe people will read that before they post.

 

Looks good to me...it all makes sense. Like Stu said...hopefully it gets used so the same questions quit being repeated. Should be a sticky IMO.

 

You stated that Mono amps run only 1 speaker. That might not be the best way to note that. I am pretty sure you can run multiple speakers off of a mono amp, it just splits the power each time you add a speaker. Thats my understanding anyway, i don't own a mono amp.

 

Maybe reword to state that is has 1 output channel? because yeah you could wire a set of subs down to the proper resistance and be just fine.

 

wwhooops
yeah my bad lol
fixed

 

Something you did not mention...Alternators and Batteries.

I plan to install a pretty killer system in my car sometime in the near future. I do not want my lights to dim so what kind of aftermarket alternator would you recommend? Also I should add another battery to my car, can I hook it into the alternator so it always charges? Can I run 2 batteries on one alternator? I suck with electrical work!

 

very nice *borat*

sticky ftw

 

Unfortunately I cannot really suggest anything for alternators, I simply had mine tuned to take 135 amps (I wouldn't suggest it because it strains your alternator a bit more than it would to get a high output one). One thing you could also do is put a smaller pulley on your alternator so it spins faster.

You can add another battery if you'd like, make sure you run it to your primary battery, though, in parallel wiring, not series.
You can run two batteries on one alternator, but at the same time you need to take into consideration how much your system will drain before you can consider whether or not you need to upgrade alternator/battery system