I've been spending some time on a few car audio forums and a few things pop up frequently, namely someone wants a killer stereo but knows nothing about how to approach equipment selection. So they throw up a generic "What do you guys think I should buy?" type thread and usually the end result isn't very informational, and most people just gravitate to naming Brands they like. I like how everyone tosses out speaker model suggestions and says "deez rawk", but nobody gives specifics on why. I'm not saying don't buy any of these, I'm just saying you should know why you are buying what you decide on. Choosing audio gear shouldn't be akin to throwing a dart at pictures of products and going with what you hit.
If you have not heard the speakers you are considering, you are purchasing them on faith. Faith in the brand, faith in the numbers they present in the specifications, faith on the recommendations of others. You also have to consider your application and listening habits. Often what you want to go with doesn't reconcile with the demands of the system you have or plan to have eventually. Too often people choose audio equipment with cost as their primary consideration, resulting in mediocre satisfaction in the end result and possibly costing more in the long term due to re-replacement in an effort to more closely meet original expectations.
You are better off slowly building your sound system than running out and buying what you can afford right now. The only exception would be if you are an experienced builder and can rely on your familiarity of components and installation criteria. Basically, don't go full retard unless you know what you are getting into.
Okay, that said, I can tell you about things I consider and look for when deciding on replacement sound system equipment.
First I decide on how full retard I want to get with the system I am building. Am I eventually going to replace everything? If so, you should decide how loud you will want your sound system to be. This will determine how much power you will need and how much power handling you will require in your speakers. What kind of sound quality am I looking for? Generally the more sick nasty you want to go, the more separate components you will need (2 way? 3 way? 1 sub? 2 subs? spaztastic 4 subs?). Then you have to reconcile this with your budget and start compromising. If you refuse to compromise and are on a limited budget then it will just take you longer to finish the sound system.
Good, Fast, Cheap - Pick two. This is generally true.
Head Units - Mostly what I look for here is features. If you plan to amplify your system the power output on your head unit is irrelevant. What you are looking for here is signal to noise on your inputs. CD signal to noise under 100dB sucks in my opinion. All things being equal you should pick the 100dB+ s/n head unit over one that has 90dB s/n. Line out voltage is important to some people, I say it's a factor but it really depends on the amplifier you choose. My HU puts out 2V on the line outs, which is fine with me because I bought a super quiet and very powerful amplifier. So I made a compromise on this one stat in the head unit, but made it up in the amp. Something else I consider heavily is THD%. The lower this number is the better. I like to see this number under .1% and I prefer that it be under .05%.
Amps - A lot of the same numbers apply here. You want a high signal to noise and a low THD%. This is where you decide how loud your system will be kind of. Good clean 50W per channel is adequate for a lot of car systems but that doesn't leave a lot of head room either. If you listen to music that is very dynamic you will want a lot of head room.
Let's say you want to be able to hit 100dB spl, which is a pretty reasonable number and you buy an amp that gets you there somewhat comfortably, but you listen to this really obscure ambient tribal shit that has wispy crap in the background and fairy footsteps in the leaves, then pounds out some kind of ritual drum beat dance climactic thing, you'll want to hear the full range of that quiet to loud which spans let's say 40dB to 100dB at your capable volume level. Well, a car is a loud ass piece of equipment when you are driving it. I bet at highway speed you're hitting at least 70dB worth of road noise. What's that mean? It means all that quiet fairy footstep shit is getting drowned out by your vehicle. So you turn it up to hear that stuff and it gets loud when the dynamic stuff picks up and then you have a distorted fart because you've reached the limits of your amp and your speakers are flopping around like an NHL goalie that doesn't know where the puck is. This is why I like good solid power in an amp and not just enough to get the job done.
Speakers - Okay this is where a lot of people put the cart before the horse. You run out and buy speakers and they are rated for 2-60W, then you get an amp because you want it loud as shit, that will do 100W+ per channel. Suddenly you don't have enough speaker for the amp you have, and you've wasted money on power you can't use.
Now this isn't necessarily a huge mistake as long as a few things are understood. One is that speakers can typically handle peaks of power greater than their rated handling, and you will be more likely to blow a speaker by driving an amp too hard if it is underpowered because amps can typically produce more than their rated power (up to double) if you run them into clipping for more than a very short time.
Just like amps however, if you can, you'll want to build a little head room into the system. If you never clip your amp and you are running 50W and you buy a 60W speaker then you will be perfectly fine. You will want to get the most out of those watts however so generally you want to buy a more efficient speaker. Anything 90dB+ SPL is pretty good. The higher this number, the more SPL you can get out of your speakers at lower power.
Frequency response is another consideration. The human ear is capable of hearing tones from 20Hz to 20kHz. Your door speakers won't be able to do this, which is why people add subwoofers. You want to plan an efficient transition between these frequencies though, and keep in mind that when a manufacturer says a speaker does 50Hz to 22kHz, that's usually not a flat response, meaning there is a point at which certain frequency ranges become diminished. I like to think that speaker is probably fairly flat between 80Hz and 20kHz and the low frequency rolls off somewhat down to 50Hz and then it takes a nose dive. Plan to add a subwoofer that will cover the roll off if you want to have low bass, and try to relieve the burden of reproducing those lower frequencies in the door speaker so it can concentrate on what it's good at by using the appropriate crossover settings (something you will need to look for in either your head unit or amplifier settings).
Okay I know this was a bit long but it should give you a decent idea of what shopping for a sound system for your car entails. Try to look at the larger scope and how the pieces will fit together as you make your selections, and the end result will surely not be disappointing.