I don't know much but Subwoofer will certainly help. If you check out my thread, he posted a head unit, 4 speakers and they all came to under $200. They sound awesome but could definitely use a SUB to clear up the sound in the speakers and get better bass. I believe that he will be able to get you a good setup posted for under $450. Also use this, it helps!
1: What is the Year, Make, and Model of the vehicle.
2: What is your budget?
3: What exactly do you want your outcome to be? (Good sounding music, or loud bass?)
4: And specific sub sizes that you prefer more than others?
5: How much trunk space do you want to use up?
6: What type of features are you looking for in a CD Player (aka Head Unit)? (Ex: USB, AUX, BlueTooth, etc)
7: What type of music do you listen to?
8: Is your stock stereo system amplified? (You may or may not know this)
9: Do you have Steering Wheel Controls? (Volume/Seek on the steering wheel)