In my opinion, if you are planning on upgrading the system yourself then you are much better off not going with the Premium Sound System (DSP). The reason is the DSP will limit what you can and can't do, and since you are likely going to add your own amps and subs, why spend the money on the DSP system in the first place.
The number of speakers in the cabin area are the same. The only difference is the DSP system adds two subs in the passenger side rear compartment. The subs are very small, however.
As for replacement speakers, here's some good examples of ones to consider
http://www.xoutpost.com/articles.php?...icle&artid=154
http://www.xoutpost.com/articles.php?...icle&artid=146
http://www.xoutpost.com/articles.php?...ticle&artid=43