make sure you get an air tight seal between the speakers, brakets and the door
the amping thing helps alot with distorsion, when you get in your car now and turn it up you can hear the speakers starting to distort, and it sounds shit right?
what that really is, is your headunits amp distorting and the speakers playing it back to you, you have to have some really shit speakers for the headunit to overpower them.
most headunits reckon theyre 4x40rms upwards, but in reality, if you used a amp with only 2x25rms(and it was a decent amp though) the speakers would play louder, last longer, and sound much better, you get more 'depth' in the sound, loose some of the tweetery harshness, and gain some mid range sound
best bet is to look around for sub, and a tri channel amp, or a 4 channel, and use the amp to drive the sub and front speakers, and throw the rear speakers away
them rainbow comps i got are only £105, and for that price they are quite amazing, Im getting quite focus'ed imaging off them, and a lovely warm sound, a audison tri channel amp can be picked up for about £250, then theres plently to look around at subs, just stay away from explod, vibe, splx, fusion gear seems cheap and nasty, and most of the rocky fosgate shit(that stuffs gone really downhill)
or best bet, decide on a budget, go on talk audio, post what your budget is, what music you listen too, and wait for the replys, then you'll get a ton of different opinions, most of them bias(I cant say Im not bias neither though), but it could help you find the bets deals