refer to Primus' post because he is right on with the quality of every component in the system will affect how the others will sound, applicable even to the wiring. I've had an amp fried becuase the RCA cable i used was cheap and it split and alowed moisture in which crossed the leads making the subs clip which i think may have also damaged one of the voice coils inside. I've since learned and made sure the wires are of the best quality possible. IXOS makes a great quality wire, and as a result they are very expensive. I think its about $100 just for the RCA cable. Now you dont have to go that expensive if you're on a budget but when my crappy RCA failed it left a $1000 amp fried.
As far as amps and power ratings are concerned, as Primus stated, RMS power is the figure you want to be comparing because it is the amount of power that an amp makes below a certian level of distoriton I believe its something like >.05% thd. (may vary depending on manufacturer) generally cheap japanese made amps like sony, kenwood, pioneer etc... are rated in MAX power. which can pretty much be any number the amp can produce even if it means that the distortion level is bad. Im not sure exactly and im sure it depends but I think RMS power is generally 1/3 of peak power. So JL amp rated at 300 watts would be somewhat equal to 900 watt sony or kenwood.(generally) I personally have only bought Zapco amps but i think JL audio makes great amps too. I think most american and europian derived amp producing companies rate their amps based on RMS while companies like sony and kenwood trying to get the general public inticed by the "higher" number on their amp. Just dont think a 600 watt sony amp is better than a 300 watt JL.
When it comes to speakers the variety is even bigger variety in what to look for. The power ratings, like in amps, will tell you how "loud" the speaker might be. again look for the RMS becuase thats how much power the speaker can handle before it starts distorting the sound. you start pushing it past that with shitty peak power and its gonna be blaring loud and sound like shit. im some case its not even that loud, just shitty sounding. the other big part about a speaker is what its made out of. the rubber or material on the edge, the cone, the casing right down to the qaulity (and size) of the magnet and wires for the coils. I've used Infinity for a few years and was never disappointed in the qaulity of the sound. I think Cerwin Vega makes some decent speakers as well but i havent had much experience with them. JL is always a safe bet too.
Im sure the components that cost koncise $800 were somewher in the range of MB Quart. or perhaps those.
Headunits are a whole other story. so many variables to making a good headunit. I dont want to make this too long so just keep in mind price does tell a big story. like in most cases, you get what you pay for. You want a better sound you're gonna have to pay more. A high voltage pre out is almost maditory for running a nice crisp sounding system ( i think its pretty standard on most mid range decks) and then just the qaulity of the deck will determine how nice the sound will be coming out of it. In my opinion, I dont bother looking for decks with the amps built in (the 4x60 watt or whatever the deck is rated at. I go for decks that just have straight pre outs (this was a few years back already, not even sure how many decks are still coming with speaker outs)
this should be enough reading for a few minutes, I know its been enough to type,
but once you've considered these thoughts start looking around at what different companies have to offer and just read. Thats how i learned. lots of reading on how amps make their power and how speakers decipher it and what makes the qaulities you want in a sytem better or worse. and so on.