hello, first time post here. I need some help and advice about upgrading my speakers, as i'm a bit of a novice!

Basically, I'm after some speakers for the car with the emphasis on sound quality and not loudness, I'm not after a full on boy racer soundsystem. I know the speaker sizes (I think), but I don't have a clue whether I'll need to buy an amplifier with the new speakers and some tweeters as well, as there are small speaker grills next to the door handles. Also I need to a buy a new head unit, I assume they should be compatible with most speakers?

Also the many types of speaker types like: 2 Way Accelerate, 2 way component, 3 way acclerate and 2, 3 & 4 way co-axial, tri-axial, 6x9's, Component set, it is getting all too confusing when I'm trying to find the right speakers.

So any help would be greatly appreciated, thank you.