I'm going to show you how my stock Buick Skylark is going to become an all out db dragster without building a wall. I did everything myself except for installing the amps which were done by a couple of different install shops. I came into car audio knowing nothing, and I am now learning more and more about it everyday. I hope to hit 150 Db legal from the trunk of my car by Y2K, many people have said that I am very optimistic for thinking that I can do this, but I don't care. I want my system pounding so hard that it sounds like someone's in my trunk trying to kick their way out! I will NEVER give up until I reach my goal, and I'm going to let you watch as I struggle to reach my goal. It may take a while but I am going to get there!
My 91' Buick Skylark
My first upgrade was replacing this huge factory radio that didn't even have a tapedeck, with a Pioneer DEH-345 cd player. I tried to install it myself and cut the factory wiring harness off (BIG mistake) not knowing that there were kits available. Somehow I managed to put it back on.
Don't Laugh! I knew nothing about car audio when I started fixing up my car. For the low's I had a complete Pyramid system: Pyramid crossover, 600 watt Pyramid Reno series amp that had a 20amp fuse(more like 200 watt), and a pair of Super Blue 12's in a ported box. The guy who installed it did me wrong too, he ran all of the wires on one side of the car(power wires and RCA cable) and cracked the door sill.
I replaced the factory Delco 4" dash apeakers with some Cerwin Vega Stealth SS-402 coaxials and bass blockers(which never seemed to work). I never knew highs could sound so sweet.
I wanted to get rid of that Pyramid stuff so I went ahead  and bought an MTX Blue Thunder amp (Pro 504)and four JL audio 12w0's. With 2 12's I hit 136.5, and with 4 I hit a 140.
I bought a Pioneer DEH-P6000 to replace my old cd player. I installed it myself. I don't think there's any cd player on the market better than this for the same price ($220).
My Pioneer CDX - P650 cd changer that I installed under the driver's seat myself. I had to install it so it faced the rear of the car because there was a big hump as you can see.    
I finally got my Rockford Fosgate Power DVC 15's! they weigh in at just about 30lbs. a piece. all i have to do now is get my box built, but 2 Power 800a2's, get my alternator, and a couple batteries, and ill be done.
my 190 amp ohio generator alternator, i got it installed by a shop that did an amatuer job, they didn't even upgrade the wires going to my alternator. 3 hours after it was installed, my car died on me. It turns out that the alternator had burned up the wires in my car. I almost had to junk my car. Luckily the wires weren't burned that bad. I took the alternator out and put in a factory alternator.
Here is the box i built in its unfinished state. I decided to sell the Power DVC's and get 4 shocker 12's sealed. I was going to mount my amps on top of the box, but I decided to mount them somewhere else after everything was done.