Steve Deckert (of Decware, if any of you know of that great company) wrote a wonderful guide to car audio. If you plan to purchase anything, or upgrade your system at all, I suggest you take the half hour or whatever to read through all of this online article:

http://www.decware.com/secrets.htm

It will probably save you a great deal of trouble in the long run.

I know it has saved me a great deal of time and money.

Eric