There is much discussion on this in the Custom Audio Section
. I believe there are even a few "sticky" threads along this topic.
Mostly it comes down to personal preference. You can spend upwards to as much as you're willing to spend, but it mostly comes down to a decent set of speaks and an amp or two, along with install.
A decent set of 3 or 4 way all inclusive speakers can run you as little as $75 and an amp perhaps a couple of hundred. Much depends on your own personal desires for an end result.
I've always been partial to Pioneer speakers, but that is just me. I realize that they're middle of the road speakers for the most part, but I've always had good results with them.
good luck, hopefully someone will chime in that has more specific info for you