Re: Mazda3 question
zako's post is pretty thorough, but incase tl;dr
5 channel amp is probably easiest route to power 2 sets of components up front, 2 coaxials in the rear, and a sub. a powerful 5 channel is also expensive.
mb quart has some decent budget equipment(if you are on a tight budget). a 4 channel(for speakers) 4x80w + 1000w monoblock(for sub) can be had for ~~$220
you will need to change out the HU and get one with RCA preouts for you to send signals to the amps. stock unit probably pumps out 15w rms to the stock speakers anyway.
Stock Pioneer 4" components
Hifonics Brutus BRZ1700.1D
cruddy prefab ported box----new box otw