Most anything is better than stock, so its worth it. Im not gonna say you 'need' an amp or HU. But I will say that yes, replacing speakers is an upgrade. But giving your speakers steroids (amp), will be the biggest upgrade, in SQ and volume.
Most go with a 6 1/2" component set but that would involve mounting of tweeter. Also, installing a component set w/o amp/HU wouldnt be much of an upgrade on SQ.
For just replacing the oem, stay with 6x9 to give more range. There are many posts and how tos on replacing the 6x9. The main concern on these speakers is the top mounting depth (TMD). Too deep of a speaker will not fit between the door grill to the window. I want to say 2 7/8" is the deepest but im not sure. Adapters/spacers are needed.
You will get the most info by running a search on your questions. There are several on here on how to exactly remove your doors and replace your speakers, step by step. This forum has a wealth of knowledge already imbedded, take advantage of it.
