I want to upgrade my car's current audio system from top to bottom, including a new headunit, amp, speakers, subs the whole shebang. I only want to use high quality products, and I will get it professionally installed since I don't know jack about electrical things.

Well, I'm guessing this won't come cheap, so I can't go right now and buy it all, so my question is would it be better to buy a few things now, a few after that, a few things after that, etc, or to save it all in the bank until I have enough to buy it all at once?