Don't even bother with the rear speakers right away, invest your money in good front speakers with an amp to drive them, that is where the majority of the sound comes from. Keep in mind that sound is measured on a logarithimic scale so 4 speakers are NOT twice as loud as 2, adding the rear speakers will provide better sound for those in the backseat, but not a huge difference to you the driver (Unless you do a lot of biznuss in the backseat ; )