don't listen to them. If 300 is your budget, go to walmart, I saw a Pioneer stereo set there that had a head unit, 6x9 rears and 6.5" fronts for like $200, that'll get you a good head unit and ok speakers, then spend the remaining $100 on the 4x6 and adapter for the stereo.
Changing the speakers will help with sound, but the head unit is still crap in almost all cars.
I ran a system in my old Mirage. Pioneer DEH-3000, Infinity ref 6.5" in the front doors, kenwood tweets on the a-pillar, Alpine 6x9's on the back deck and 2 fosgate punch 10's in the trunk powered by a 350W amp.
The speakers inside the cabin ran off of head unit power. That car was loud, I could turn it up so loud that you couldn't sit in the car. And it was clear as day.