My suggestion, with your budget in mind would be something like this:

Sound Ordnance™ P-67 6-3/4" 2-way car speakers at Crutchfield.com
+
Diamond Audio D192i 6"x9" 2-way car speakers at Crutchfield.com
+
Sound Ordnance™ M-4050 4-channel car amplifier — 50 watts RMS x 4 at Crutchfield.com

I know its over budget by a bit, but the rear speakers are going to be your source of bass and going cheap there is going to really hurt the sound. The amp will really help you get the most out of the speakers.