In car audio, you get what you pay for. If you buy a crappy Boss stereo instead of an Alpine or Pioneer stereo, you'll probably be sending the Boss stereo in for warranty issues.
Brands that are good for the following categories are:
Amp: Top 5: Rockford Fosgate, Hifonics, Alpine, Kicker, Infinity.
Honorable Mentions: Kicker, Polk Audio, Boston Acoustics, Pioneer.
Speaker: Top 5: Focal, Rockford Fosgate, Kicker, Infinity, Alpine.
Honorable Mentions: Boston Acoustics, Kenwood, Hifonics, Polk Audio, JL.
Subwoofer: Top 5: Fi, Alpine, Kicker, Rockford Fosgate, Pioneer Premier.
Honorable Mentions: Hifonics, Infinity, Kenwood, Pioneer (Basic Pioneer).
OEM Processors: Rockford Fosgate 360.2 Processor
On a Budget: Hifonics Processors.
Wire: Kicker and Rockford Fosgate.
Also, stick with name brand companies that follow the AWG system.
Head Unit: Alpine, Pioneer, Kenwood.
The only place I recommend to buy any of these products are Crutchfield.com and the actual company's homepage that makes the products so you can get your warranty with the product.
When you upgrade anything in your system, it's best to upgrade everything, because every component will compliment each other to make the best sounding system.
A rule of thumb when designing a custom system is that when you add (a) subwoofer(s) you need to follow the following formula when going for SQ (Sound Quality) instead of SPL (Sound Pressure Level): Total RMS (Root Mean Squared OR Constant Wattage) of all speakers should be double to figure the RMS that the subs should be. i.e. If you have a 4-speaker system that has 50w RMS per speaker, then you'd 50 x 4 giving you 200. Then take 200 x 2 which gives you 400. That means that your system should have a subwoofer(s) with 400w RMS.
You should also tune your system for optimum output. Here’s what you do: Turn your vehicle on (make sure you have proper ventilation, too.) Next, put in some earplugs (if you think/know your system can hurt your ears, go ahead and put in some earplugs just to be safe.) Then turn all your settings down (on your head-unit (and amp(s) if you have them.) Proceed to turn your volume all the way up. Next, start setting the gain, filters, settings, etc. on your amp(s) if you have any on your system. The next step is to start setting your head-unit. Set the bass, mid, treble, and any other filters available on your head-unit. You can use the subwoofer setting to apply more/less bass to your taste when listening to certain types of music. Some systems even let you choose what size speakers are being used. *Also, the gain for the amp(s) should be the pre-amp output voltage listed in your head-units manual (follow this number and match it exactly to your amp or you’ll risk damaging your system and your sound quality.)
Some systems require a power upgrade. There’s three things to do and it’s imperative that you do it in the order I list them. First, the Big 3 Upgrade. The Big 3 Upgrade replaces the cable from the positive terminal of the alternator to the positive terminal of the battery, the negative terminal of the battery to the chassis ground, and the engine ground to the chassis ground. 0 gauge wire or larger is preferred. Second, you can upgrade your battery. I suggest Kinetic batteries, but Optima will do, too. Third, an H/O alternator (High Output) will provide you with the proper amount of amperes to properly power not only your A/V system, but also your car’s OEM electronics (most people forget about powering those components, too.)
You can do all this on your own. I can give you links to diagrams that I've made to help you install your system and if you order from Crutchfield.com (which I highly recommend) they'll send you a Master Sheet with everything on it.
If you go to Crutchfield.com you can click on "Outfit My Car" and enter your car's info, then Crutchfield will show you all the products that will fit/work with your car.
If you need any help just e-mail if you need help!!!
Lastly, as with everything in life: Reliable, Cheap, Performance: You can only choose two.