Speakers are nothing without a decent amp to run them. Just switching out the speakers and running them off of a new headunit will never sound as good as a system with an amp.
Running a 3 way component system up front, 6.5" Hertz HSK 165 Components (Low range) 4" Boston Pro Components (mid range) and 1" Boston Pro Tweeters
and it definitely kicks hard.
Oh also running a 3 way component rear system
5.25" Boston Pro Components (low range) 4" Nak components (mid range) and 3/4" Boston Pro tweeters
Rear speakers are Definitely needed, try fade all the power to the front and see how it sounds. Rears are needed for sound fill, also are good for back seat riders. You don't need anything expensive in the back, if you are only after sound fill.
Running a 12" JL sub as well. here's my thread:
The Building of a Custom Walnut Sub Enclosure.