Here is my contribution for those who want to put Amps & sub in thier car.

ARE YOU WILLING TO MODIFY YOUR CAR?

Are 15-inch subs going to fit in a small trunk? Probably not. Can you still put a great system in a small trunk? Sure!
Carefully examine your car and physically measure how much room you have in the areas in which you want to install electronics components and speakers.
Try to envision what kind of work is going to be required to put the pieces where you want them. Don't bite off more than you can chew. Think about how long it will take. You may have to forgo driving the car untill your done.

CAN YOUR VEHICLE HANDLE THE DEMANDS OF MULTI-AMP SYSTEM?

Your vehicle's electrical system is designed to do one thing very well: Supply the car's diff. components- headlights, air conditioner, etc.- with the needed current and voltage to operate properly. Luckily, some manufacturers design their cars electrical system with a reserve capacity that is well-suited to supplying power for a sound system.
Most alternators are capable of producing 55-65 amps. This is adequate for sound system packing up to 350 watts RMS.
Most German automobiles have charging system rated up to about 90 amps. This will sufficient for systems up to 500 watts. Some HONDA and TOYOTA, on the other hand, have alternators only rated for 35 amps. This will only support about 125 watts in amplifier power.
If you want more power than your alternator can service, you will have to look at having your alternator rebuilt fo high-output. Or you can buy in a high-output aftermarket alternator.
Installing a second battery wont help. The only reason to do that is to play your stereo for long periods with the engine off. On the other hand, you could be asking for trouble if you keep the volume cranked up. If your system needs more current than the alternator can supply, it can grab current from the battery- up to a point.

ESTIMATE YOUR CURRENT DEMANDS

First, add up the fuse ratings of all your amplifiers. Now figure your amflifiers total steady-state average output(divide total RMS wattage by total peak wattage). Then multifly your fuse rating total by your amps total steady-state average output. This gives you a rough estimate of how much current your sound system is going to require.

HOW TO ESTIMATE YOUR ELECTRICAL SYSTEMS RESERVE CURRENT CAPACITY.

Take your alternators amp rating and multifly it by 40 percent. This rough figure will give you a baseline for about how much reserve capacity your cars electrical system has.
For example: your alternator is rated for 90 amps.
90 amps x 40 percent= 36 amps left to power your sound system. If this figure is higher than your estimated steady-state current draw, your in good shape.

TO BE CONTINUED......