Make sure that you keep in mind that most aftermarket speakers have a lower sensitivity then stock speakers. This means basically that the stock speakers, driven with 22 watts RMS power will sound louder then aftermarket speakers with the same power.
While the aftermarket speakers will definatly sound better, they will probably be slightly quieter then factory, but this can be solved with an amp.
In my '93 I have stock front speakers and Infinity Kappa 6X9s in the back. There was quite a drop in volume going from the stock to the Infinity speakers in the back. Not a difference that would make me change them back, but noticable.
1982 Cutlass Supreme - mild SBC, tons of fun! The summer toy
1996 Silverado Z71 - the hauler
2002 Bonneville SLE - His / project
2003 Bonneville SSEi - Hers / never ending project black mirror