Okay... It depends on what you want and what system you have now...If you
have premium stereo in your car... which is with speakers  by the door, you
can get away with just changinge the head unit. It also depends on where you
are... HD radio is a good option to have and if you want satellite too...
look for unit that have both.

If you have standard radio system, no door speaker, then I'd spend a little
more money and install external amplifier... spend extra money for front
speakers... get the components. Components are expensive but can be had
cheaply on ebay. They are worth every cent. Of course by adding external
amp, you have to run new power wire to amp... speaker wires, and RCA
wires... it will add up. But again... everything is worth it.

Partsexpress.com has alot of accessories you need to make the whole thing
work unless you want to hire someone to do the work for you... look for some
buddy who got experience to help you out.

The general design of a sound system that I like is what is called a 3
channel plus one.  I run my front speakers and a subwoofer off a power
amp and run my rear speakers off one of the pairs of outputs from the
head unit.

I like JVC head units.  The model I have in my 1982 300TDT is no
longer available, but here is one with the same capabilities, plus the
ability to control and iPod from the unit's remote.

http://www.circuitcity.com/ssm/JVC-In-dash-Player-KDPDR30/sem/rpsm/oid/172698/ca
tOid/-13089/rpem/ccd/productDetail.do

I would recommend getting some front speakers capable of handling a
lot of power.  I have Pyramid  4094 4" speakers in the front.  They
will likely fit your stock mount space.  They handle up 200 watts and
sound great with rock or jazz or classical.  $19.95 at eTronics

http://www.etronics.com/product.asp?stk_code=pyr4094
For the rear I put in some  Kenwood KFC-6990ie 6" x 9" 2 way
speakers.  I like Kenwood because their bass response goes down to 28
cycles for good low bass. A bit pricier, they run $64.99.

http://www.etronics.com/product.asp?stk_code=kenkfc6990ie

And for a power amp you can't beat a Panasonic CYPA4003U 1000W 4-
Channel Power Amplifier.  I like a four channel amp.  You run the
front pair off one pair of outputs and then "bridge" the other pair to
a single high powered channel to run your sub woofer.  So that is
called a 3 channel setup, the two sides of the pair you run to the
front speakers and the bridged single output to the "pump," the
subwoofer setup.  You want a power amp that has two pairs of line
inputs and also has a preamp out.  The Panasonic I recommend has all
these features and a special bass circuitry that gives a nice, full
natural bass.  It's great, like the Pyramids, for rock, jazz or
classical.  You can find one for less than $100 if you do a Google
search for one.  I found one for $89 a couple of months ago.  I have a
Baja unit in my 4Runner and an Alpine in my Benz (very expensive,
nice, but not that much better).  The Baja kicks a.s too.  They are
maybe $10 less than the Panasonic, but not nearly as nice.

http://www.etronics.com/listproducts.asp?order=stk_price_a&catid=8063&store=&Mak
e=&Model=&type=&kit_id=&prod_type=&manaf=Panasonic&image1.x=11&image1.y=19&cid=1
A70D08821414542911A247B236197B5

And for a sub woofer I like CDT Audio speakers.  http://www.cdtaudio.com/versabox_subs.htm
Again, I have one from their 2006 product line that is slightly
different from what they now offer, but an equivalent speaker series
is the CL-EFCast Alloy Audiophile Versabox Subwoofer.  These CDT
subwoofers put out great smooth bass and offer sound reproduction
definition for jazz and classical and can also thump out the bass line
on a Wilson Picket or Led Zepplin tune!

http://www.cdtaudio.com/versabox_subs.htm

You can pay a lot more for one of the more expensive series from CDT,
but the sound can't be much better.  I make my own subwoofer boxes out
of solid hardwoods.  I have a maple box in my 123 Benz, a mahogany box
in my Corolla GTS and a pine ( I know, pine isn't hardwood) maple box
in my 4Runner.  They all sound a little different.  I like them all.
If space is a consideration, like it is in my Corolla, I used an 8"
woofer.  I have 12"s in the other cars.  The 8" kicks a.s just fine.

I'd buy my subwoofer from http://www.woofersetc.com , although I can't
find the series of CDT woofers I am recommending at the WoofersEtc.
site right now.  Google the model of CDT sub you want and you will
find a good deal someplace.

I guess that's it.  I was just out in my 4Runner listening to some Led
Zepplin and that system sounds great!

There are two things that are key - the subwoofer in a good box and
the power amp.  Without those you won't have decent sound no matter
what. Pay attention to the volume of the box.  The CDT speakers come
with a spec sheet that will help you design a box or order the right
size.

A head unit with two sets of preamp outputs is essential so you can
set things up so the front track is the front track and the rear track
is the rear track.  If you get a head unit with just one pair of
outputs you will have to have the rear connected to the front and vice
versa because the single pair of outputs probably will be the rear
channel, and the line out is what goes to your amp and with just one
set of outputs you will have to pass that through to the subwoofer
bridged output on the power amp.  I have single output head units in
my Corolla and 4Runner and I really prefer the two pairs of outputs
unit I have in my Benz.

I hope that helps.  I have been sitting here at the computer waiting
for the morning chill to be out of the air so I can comfortably go
crawl under my Benz and finish disconnecting everything so I can pull
the engine tomorrow.  I am having it remanufactured after 400,000
miles so the car will have kick a.s sound and run great too.

Paul