sound system revamp

[daibatzu]

hello everyone. this is my first post. just got my new yaris sedan last week and decided to join a yaris forum. I am a bit of an audiophile and the first thing I noticed about my new car is that I really hate the sound quality coming from my speakers. But I really do not know anything about sound systems for cars.
I am thinking of buying this:

http://www.amazon.com/Sony-CXS-GT541...958906&sr=8-13

But can anyone let me know if there is anything else I will need? I can see there are speakers at the back seat and in the 2 front seats plus a CD player. Do I have to replace all these in which case I will have to buy the set in the link and also speakers for the back seats? In which case I will have to add this as well?

http://www.amazon.com/Sony-XSGTX6930..._bxgy_e_text_b

Please help. My car dealership says they will install a new sound system for free but I need to buy one first and don't want to make a mistake.

Many thanks,
Igwe