speakers for car help?

[Lbrown]

so i got like a toyota corolla and wanted to fix in new speakers . pretty new to all this stuff . well i just got installed a sony headunit , model CDXGT300MP. its a cheap one . But first of all does the headunit affect the sound quality much , if so im thinking of buying a new headunit but dont know. Now i just wanted to install speakers and wondered if i had to change all 4 speakers in my car . Speaker packages only come in a pair of 2. and whats with all the watts st uff . my head unit says something like 205 watts , and im lost haha. so question one is should i change my headunit , and does it cause a dramatic change in the sound quality? secondly is should i change all 4 speakers or just the front. and finally can some one briefly explain the headunit watts and speaker watt thing. thanks