imho, one requires 1) >=1 external amp, and 2) a sub to get a car stereo to sound good with whatever you have for speakers, obviously better sound w/ better speakers. And you can do it faiirly cheaply if you want.
I drive an extended cab pickup truck. 2 amps sit inside the center console, one is a Precision Power amp powering a JL Audio 10" sub in a sealed box (great stuff), and another Kicker amp for 4 satellites. 2 of the satellites are Klipsch ProMedia center speakers sitting on my dash, one on each side; conveniently, these are 4 ohm speakers - you can pick these up directly from Klipsch for $25/ea as replacement speakers. For the rears, I recently upgraded from Klipsch promedia w/ the 3" woofer and installed 2 Yamaha outdoor 2-way speakers (6 ohm), $100 for a pair, over the rear seat. The system kicks arse! Very clean is the comment I hear most. ANd it is.
It's sourced via a 1/8" mini stereo jack to RCA L/R going to the amps. The 1/8" jack is usually plugged into a iRiver portable CD player powered w/ a cig lighter plug; the cd player is mounted on the dash face. I can also use my Tascam HD-P2 as a source, which is great to hook up to check out the show that was just recorded on the way home
Lots of flexibility and easy to change things around, once the amps are installed that is.
THis is installed in addition to the regular in-dash factory radio speakers, which I decided to leave alone so as to preserve AM radio which always seems to crap out when a different head unit is installed.
/Ken