Common Misconception #1. This should definitely be addressed first. This is the most common theme among people interested in upgrading.
I need the biggest and best hardware in order for my car to sound good.
Truth: Unless you are a serious audiophile, there is no need for the biggest or the best. Brands like Focal, DLS, Digital Designs, CDT, Boston Acoustics and other very high priced brands are only worth the money if you are competing or just need to feel like you have the best. Truth be told, most people considering upgrading their car stereo source, speakers, amplifiers and subwoofers would do just fine with almost any brand you can find. These brands include Infinity, Sony, Pioneer, Kenwood and other popular brands. These brand's entry level lines are to be considered quite listenable. This applies to their amps, source units, subwoofers and interior speakers. These selections should not be driven to 100% of their advertised capacity, but they will sound good at 50-75% of their advertised output. The higher up the food chain you go, the more willing the items will be to reach their full capacity and you will begin to hear tonality differences at higher volumes.
You will find many people who will say "Infinity Reference is crap, I got the Kappa's", but if you sat them in a room and A/B'd speakers for them, they likely couldn't tell you which was which. They may even find themselves preferring the less expensive speaker. Speaker selection is very subjective and you should really listen to speakers in person before you buy them.
This is not to say that there is no junk out on the market, because there is. It's not the sound that's so bad, just the longevity. As long as you stay with a brand you have heard of for years (like those mentioned above who make all types of consumer electronics), you will likely have a positive experience. Examples of companies I would stay away from companies like "Dual", "Pyle", "Pyramid" and the like. Some may have success with their products, but generally speaking, they are not of the highest quality and most people experience premature failure of these items.