Well imho....
The speakers in the dash would help, but personaly I would buy a good set of component speakers (the tweeter is independant of the mid range, like two seperate speakers, but made in a matched set)
This allows you to mount the tweeter up by the wing window, or the corner of the dash, etc... and gets more sound to you.
As a general rule, if you need more volume, then you need more power. Most decks (even real good ones) only put out about 30-40 watts per channel, and the power is not real clean.... its just not enough room in one little unit to do it all (play cds, recieve signals, amplify, etc..)
You would be best off (again imho) by getting a 4 channel amp, like a 40x4 would do well, and run it to the speakers, two channels to the fronts, and two channels to some in the rear pannels alongside the seat, or even in boxes on the rear seat floor facing up. (if you dont carry 4 people all the time)
This will give you more sound, and cleaner sound as well... if you find that multiple amps are a pain, then you can get a 5channel amp like Alpines V12 (or whatever they call it now-days) that has like 40x4 AND 150x1 all in one amp, this makes wiring and space available, an easy deal to handle.
As for a head unit, if you have an "i-pod" then I think you are stuck with Alpine (propriatary deal with apple), but if its just a mpeg player, then almost any newer good stereo will have an aux input that will hook right up to it.
Like my Clarrion has 128megs of memory inside the deck itself, so it can store like 4 hrs of music to listen to even if you dont have a radio antena... (kinda cool).
But anyhow, any good name brand will make good equipment, as long as you buy their good lines.... if you buy their entry level cheap stuf, guess what, you get entry level cheap stuff....
Buy a good head unit, good amp, and good speakers once, and be done with it. My system in the 5.0 is like 4-5 years old already and it still sound great, and will probably continue to for another 5-10 years.
Oh just kinda an fyi... if you hook it up yourself, one mistake I see alott, is dont run the power leads for the amp, RIGHT NEXT to the signal leads carrying the sound to and from the amp.... the induction caused by the magnetic field around the power leads will give you noise on the signal lines and make it sound cheap.
Best of luck to you,
Dave-