hi! medyo newbie ako sa car audio systems so...

my ride is an all stock corolla 93 GLi. I was planning to replace my stock rear speakers with a sony xplod 6"x9" 3 way speaker (300 W yata) that i chanced upon in the mall. my head unit is the stock apline unit (I THINK 25w output??) with its corresponding CD changer. i just want louder and clearer music not really the boom-boom type. my question is is this set-up possible, meaning can i just buy the speaker and connect its wires to the head unit? or do i still have to buy other additional stuff such as an amplifier, etc. budget? let's just say extremely nice audio systems are not an immediate priority. kung baga, minor upgrade lang talaga. thanks for the replies!