Sound System Questions, 5 quick questions :) Options

[JonCars17]

I have a few sound system questions.

I'm thinking of adding a sound system to my car by the end of May which gives me enough time to save up.

Q1:
What is the point in putting a Sub (8") in both side doors?

Q2:
How much should I roughly look at spending for the system (components in the front, 2 speakers in the back, headunit and a sub 10" with an amp)? I'd like to get one for qaulity and not the extreme bass.

Q3:
What should I look at when choosing a system? the Sensivity aka the dB?

Q4:
How do I calculate how much amp I need

Q5:
What can I do to stop the car from rattling once it is installed?


I'm a n00b when dealing with cars -_-.

[lilsteeg]

So how is this?

CD Player:
Alpine CDA-9856
Price: 200
RMS Watts: 18 Watts, Peak Power 50 x 4 channels

Component Speakers:
Alpine Type-R SPR-17LS
Price: 250
RMS Watts: 70, Peak Power: 350

Speakers:
Infinity Kappa 652.7i
Price: 150
RMS Watts: 75, Peak Power: 225

Sub:
Infinity Kappa Perfect 10.1
Price: 150
RMS Watts: 350, Peak Power: 1400

Amp:
Infinity Reference 611a
Price: 350
RMS Watts: 456 watts x 1 at 4 ohms/657 watts x 1 at 2 ohms

thats a good moderate system there