MC-
   I put Polk 2-way 5.5" up front and they thump real well for the price.
The rear is more difficult, but I put in Infinity 4" (there are two
different models, with different ratings, but the factory unit is only 20W
RMS so the smaller 35W RMS version worked for me).  The front door panels
didn't need to be modified themselves, but the mounting hardware is
integrated into the factory speaker, so you (or whomever installs it) will
have to destroy the factory pieces.

   Overall I think it sounds quite nice.  Sooner or later I will go to a
better head-unit with XM and MP3 and optional inputs.  At that point I will
put bass blockers on the 4 main channels and install my 12"-500W pioneer box
into the trunk.  Anyways... My opinion is that speakers are worth it and the
factory unit isn't bad, but still a little weak.

Mike

I could only fit 4" JL Audio (TL series) speakers in my '02 WRX wagon
doors, but I wanted them to fit completely behind the stock grill
opening.  No problem in the rear - they fit right in.  But the fronts
required a fabricated mount to get as much clearance between the window
and the magnet as I could.  I kept the stock tweeters in place, but
disconnected them.

At first, I also used the stock in-dash cd changer, and used high end
speaker-level converters to feed a 4 channel amp under the seat.  It was
a big improvement, but it didn't sound like the money I had spent.  I
eventually added a used Alpine head unit that disappears into the dash
with no intervention from me when the key is taken out.  It looks like I
have no radio at all, and I don't have to carry a faceplate with me.
The ability of the 7969 to tailor the sound to my liking made a huge
difference in my satisfaction of the whole system.  The bass and treble
dials on the stock unit didn't cut it.

Even with only 35 watts to each door speaker, and 120w to the sub, I am
very happy with the output levels.  I was really surprised the 4"
sounded as good as they did at the levels I was driving them at.  The
ability to vary the crossover frequency in my head unit helps immensely.
 I had to change my battery recently, and forgot to change the default
settings.  When I sent the doors 125Hz and above, and the sub 100Hz and
below, all was right again.  The slope of the crossovers are shallow
enough that very little is lost between 100 and 125Hz.

So I have a nearly stock look with much higher performance.  I am
impressed by the speakers ability to present a wide sound stage when
they sit a few inches from the floor.  I've heard many nicer systems,
but this is still quite good.

goto http://www.iaperformance.com they have spacers that let you put upto
6.5" in the front and 5.25" in rear.  Kit start at $31 for FT and RR and $16
for just FT.  If you get a speaker with a higher sensetivity( I cant spell)
you will get more sound with less power.  I like pioneer they get really
loud with very little power.  They sonud pretty good to me.  I'm old(32) and
do'nt want a bunch of noise.  Plus you can get the fronts and rears 3-way
for less than $200!!  Yeah!!!!  I hope that helps

By the sound of the other posts, it seems like the WRX is a bit more
difficult installation than my old Legacy.  I went with 6.5" Clarion
speakers all around, I forgot the model but I think they were about $100 a
pair.  Originally I used a crappy Aiwa CDC-MP3 head unit, but that
eventually broke and I ended up with a much better Kenwood unit, a MPV-622
if memory serves.  And, after my friend got married, I "inherited" his
Pioneer amp and sub.  I can never remember the model of either, but I think
that it runs between 100 and 150 watts.  All in all, it sounds really good,
except that the interior of the Legacy seems to help amplify the higher
frequencies, so I had to raise the lower frequencies and lower the high ones
to give it a more "balanced" sound, to me at least.

-Matt