@ Ryan, the best advise I can give you when dealing with a tight budget. My focus would be as follows: 1) you first focus should be your head unit (unless your speakers are shot). Figure out what you want out of your system, if you not going to be amping you door speakers then you want a unit that has a good built in MOSFET at least a 50watt. then if your going to have subs and/or amping your door speakers look at the RCA outs, how many, how many volts they are I would not go lowere than a 3V and you want at least 2 out puts. And then from there its all the extras IPOD connetctivity, USB, MP3.....ect
2) Wire- Its useally the most over looked thing when people do their systems, your speaker/power wire is just as important as your head unit. If you have a good head unit and good speeaker wire you can make a not so good speakers sound really good (Think of it this way, your speakers reproduce whats being sent to it. So If you feed your speakers crap, its going to sound like crap. If you feed them a good clean signal with good transport its going to sound like it). I would not go smaller than 18ga, 12ga is ideal. If going with a sub as well make sure you have adiquite size power and ground wire for your amp, too small of a power wire can lead to: 1) melting of your jacketing which could lead to a fire, 2) make your sub sound week because the amp can't draw the power it needs.
3) Amps- Its really hard now to know who has good stuff now since everybody is made by every body, but as long as you stick with a nambrand you'll be ok. Also do some research not all amps are created equal, one example I can think of is sony now they may have changed but several years ago sonys amps were not true to their rating. Meaning if I bought a 1000 Mono class D amp it would only be putting out around 1/2-3/4 of what it said. So just be carfull.
4) Speakers- The least important but at the same time the most important...remember garbage in garbage out, clean in clean out. When you speakers there is basiclly a good, better and best and then a good , better and best with in that. So most people if you have set up your system well meaning a good head unit and wire can't tell the difference if you have a $100 a pair component speakers or a $200 a pair component speakers. Now thereare a lot of factors in side that but as a general statement they cant. So buy your speakers to fit your music..ie if you listen to a lot of rock, rap..ect I would recomend a metal dome tweeter. If you listen to a lot of vocals, classical..ect I would say go with a silk dome. But again its your ears and music do what sounds good to you.
5) Subs- Size matters...but not how most people think. Again it goes back to what you listen to, If your a rap and r&b person I would lean towards a bigger size 12" or 15" if you are more into rock and classical I would lean towards 8" or 10". The POWER you must know what speakers spaciffically SUBS if you buying a sub that is rated for 500 watts RMS you could safely 750-1000 wattst and be fine. MFG's put rattings lower than what they can handel to keep uninformed people from blowing up their speakers. Kinda jumping back to amps for a second, if you sub has a RMS of 500 watts and you decide to put 1000 watts to it, when you buy your amp you are going to want to go with a little bit bigger like a 1500 watt. this alows your amp not to be running at 100% all the time. because who puts in an amp and turns down the gain to only a 1/4 or 1/2. by getting a bigger amp you can run it at the 1/4 or 1/2 and A) increasing its life span B) the amp is cleaner when not at 100% for normal use (doesn't mean it cant spike up to its full usage)
6) Boxes- this extreamly important, not having a properly designed box could make or break you in the bass department. I had a friend that was in the audio business and he had made a box for his sub, before I knew what size was in there I could swear he had like a 12" or dual 10's". come to find out he was running a single 8" thats right a SINGLE 8" and boy did it hit hard
if all you can get is a premade thats fine those are nice, but if possiable find someone you know/ or yourself thats good with wood and build it to the driver(subwoffer) it makes all the difference in the world.... plus they fit and look better
Well I hope this helps you make your decsions on your gear.
P.S. If all you have money for is a good head unit and speakers, I would hold off on the speakers( unless the ones you have are shot) and upgrade the wiring
then save and get the other stuff. Good Luck Sir