I upgraded in parts because I was on a budget. First came the front speakers ( they are closer to your ears ) , then the rears and finally the headunit. I would recommend speakers before the headunit, because even if you buy a great headunit you will still be playing through crappy paper speakers. When both my speakers were replaced, even with the OEM headunit it sounded GREAT

Then when I had the $$ to buy the headunit the sound went to AMAZING

My system isnt earth shattering but it has a high quality sound. Depending on your listening style if done correctly you will be satisfied without a amp & subwoofer. Pick a headunit with lots of audio options. Thats why I bought the kenwood Ihave. 8 presets, 2 levels of loudness, 3 levels of bass boost, you can set the size of the speakers you have as well. It looks pretty standard but has it where it counts....sound ! Also install some sound insulation on the doors, you will get the most out of your setup and the vibe will be quieter inside. Crutchfeild has lots of articles to teach you how to choose speakers and headunits, so be sure to read them. It will make you more informed of what you purchase and get the most of your $$$. good luck !
I recommend Polks by the way.