Actually, when I got everything together, the two 3 1/2's in the dash were louder than the other four speakers. I had to turn the gain up on my amp so that everything sounded equal. They are closest to your ears and the sound bounces off the windshield right at you.
Don't forget if you go this way to put some capacitors on the small dash speakers. This will prevent the lower frequencies from reaching these speakers and causing them to distort or possibly blow at high volumes.