2007 Cobalt Audio Setup
The place I learned how to setup all this stuff was caraudio.com, great forums there that can teach you everything you want to know.
I don't know how anemic the amp in the stock head unit is, but it would probably be fine to power your front and rear speakers if you don't want to amp them.
Sony is a terrible brand for car audio, look around a bit more if Sony is the path you're considering.
And please don't quote the "max power" stats like you've done so far when mentioning your proposed setup. "Max watts" or "max power" mean nothing, essentially. Always go by RMS power for both subs and amps.