My 2cents.
Front speakers:
2 way components in the front (midbass in the doors and tweeters in the A pillars).
These are the most important speakers in the system; so audition as many as possible and pick the ones you like. Take your own music to the shop when auditioning and listen closely to the tweeter for any harshness
Rear speakers:
The least important. Leave the factory ones in there unless they're already knackered.
Headunit:
As above, CDs are pretty dead. Ipod is the way to go for convenience and control from the headunit. There's not a heap of difference in sound quality between brands and pricing is usually dependant on what feature you want. Look for something with 2 pairs of RCA outputs (aka pre-amp outputs): this gives you more options for system expansion should you want to add anything later. Look closely at ipod control, factor in any adapters you'll need into your budget.
Subwoofer:
One of the best bang for $$ improvements. Doesn't have to be ground pouding, it will add the bottom end and fullness that your front and rear speakers can't physically handle. 10" in a sealed enclsoure is a good start point. Sealed boxes are smaller tahn ported or bandpass boxes so they save boot space. They're also the best if you drive on dusty roads. Recomend a 10" single 4ohm (or dual 2ohm wired in series) that will handle 250rms +
Amplifier:
Ignore any power rating that doesn't say RMS. Max power and peak power are marketing hype and should not be trusted. I'd look for a 4channel amp with approx 80rmsx4. This would give you 80rms for each side ion the fronts and bridge the rear channel to run a sub around 300rms.
Capacitors:
There is no need for a capacitor in a car audio system. Do not buy one cos they're a complete waste of money.
Wiring:
Don't skimp on earth wiring. Grounding amplifier to chassis and chassis to battery is hugely important. Welding cable is alot cheaper than specific car audio cable and is as good if not better.
