Going on what ^^^ said... I would recommend upgrading your door speakers first! I was able to fit the Infinity Kappa 682.7cf 6x8s in my front doors, even though multiple installers said that those speakers would not fit! Although changing the head unit and speakers made a difference, nothing really came alive until I gave my door and deck speakers 100 watts RMS each
Regardless, my aftermarket setup is a huge improvement over stock but there is nothing like the sound of some great component speakers. I will be replacing the Infinity Kappas some time in 2009 and probably going with a DIY component set.