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 -_-.

[JonCars17]

so this is how I should set it up right?

1st:
CD Player and Rears (factory)

2nd:
Sup and Components on the amp

What happens if I add rears, does that mean I have to add another amp because on the amp I'm getting only has 2 speaker connections on the back.

see:
http://akamaipix.crutchfield.com/products/...RDM105-i-1.jpeg

I'll most likely go with the Type X components thou but I doubt I buy them this week it may by a week or 2 for that.

This is basicly my problem now, this and enclosure. I just don't know which type of enclosure to get, type: 1, 2, or 3.