Upgrading Speakers, Completely Lost

[narfcrunk]

I've tried combing through these forum posts but for the most part, the terms and acronyms are tough to sift through and I'm not sure what would suit my needs.

I drive a 2011 4 door sedan with the stock head unit and speakers and normally listen through a bluetooth dongle plugged into the aux port.

I would like to install new speakers for all 4 doors and for the ones sitting behind the headrests of the back seat but I don't know which aftermarket speakers would be best for me.

I'd like to keep costs down, but beyond that I don't have any super high powered needs. I just want louder, crisper music than the current speakers offer. Bass isn't super important to me, but a little would be nice.

Thanks for all your help!

PS:
Things I'm not sure of:
Do I need an amp or could I wire the new speakers straight in the place of the old ones?
What size are the speakers behind the rear seats headrest? I'm pretty sure the other speakers are 6.5"