I did a bit of research on this back a month or two ago because the music wasn't loud enough when the windows were open on the highway. I want better bass response and less distortion at higher volumes.
We can get line out converters for our stock head unit, run them to an amplifier, and then to the speakers. Doing just the amp will yield a decent improvement, and a decent amp will cost ~$150-200+
If the amp isn't enough, speakers can be put in, component speakers up front, and rear coax speakers are probalby fine. They will range somewhere in the 125-200 range for front speakers that are nice, and rears will be cheaper (if you do only coax).
Feel free to PM me about this, I got a lot of my info from i-club's stereo gurus. I figured for about $500 I could have a really decent setup without having a sub in place.
I eventually will do this, I've just been spending money on other things lately.