Audio Systems: new system questions, size speakers, channel amp

Question
Hi there,
I see that you're the man when it comes to car audio, but this is new to me, so I had a couple questions. I have a 99 Passat that I want to hook up. I'm thinking 4 new speakers and an amp for 'em. If I buy 4-channel speakers do I need to buy a 4 channel amp? Should I even buy 4 new speakers? All of my current ones are fine, I just want louder/better quality, would I be cool replacing just 2?
Also, what size speakers should I get?
A buddy got 6.5's, in his passat, but i'm thinking that's too big. Can i go smaller?
I'm looking to assemble a speaker/amp combo, so let me know what you think.
Thanks for your time
Sascha

Answer
Yes, each speaker should have its own power amplifier feeding it.

You will be restricted in size as to what will fit into the spaces without major modification to the door panels, etc.

Maybe getting more sound output will be just a function of getting a bigger set of power amplifiers.

Also, I have found from experience that what most people want when it comes to more output is really more bass power.  The greatest bang for the buck in a car system is to add a really good subwoofer.  You can add it to the existing system and see if that works.  If not, then add upgrades of the power amp system and/or speakers.

I like www.crutchfield.com because you can use their calculator to determine what range of products will fit into your car and how to wire them up.  You don't have to purchase from them, but there resource and guidance is excellent.

Hope this helps.

C