OK, Digipants has it bang on - there are four channels output from the factory headunit, which in turn are split high/low for a pair of mid's and tweets in each corner of the car. IIRC on the mk4 the crossover (which splits the signal hi/low) is actually part of the mid range woofer - therefore you have two (full-range) signal wires from the headunit to the x/over (located on the speaker basket), and then two go to the mid range woofer itself, another two shoot off in the direction of the tweeter.
As such, a standard four channel amplifier (or two, if you wish to just amplify the fronts) is easy to set up, however I'm unsure of the system expansion possibilities of your specific headunit; I know for a fact you can buy adapters that will give you at least one low-level (un-amplified) output, but I'm unsure if there is an adapter that will give you 3 sets (front, rear, sub). As such, if you want to retain the ability to fade your sound (front to rear) from the head unit, you're going to have to look for an amplifier with high level (speaker) input terminals.
However, I respectfully have some recommendations before you take this route;
Before looking at amplifying your full range speakers, look at adding a subwoofer. Your standard factory fit speakers are never going to produce earth-shattering low's, even when amplified. Low range notes are produced, in essence, by moving a lot of air, hence a large cone area or a long cone exertion within its chassis are required - your standard speakers are not capable of either.
Now, when you add a sub to take care of the low stuff you can then filter out these low-end frequencies (using capacitors) from your full range speakers. This will allow you to drive your factory speakers harder before they start to croak; Make them twice as effecient and you can work them twice as hard.
A sub amp can easily be added to your existing headunit and would be installable (even for a novice) in a couple of hours.
Allow the balance of the day for filtering out your existing speakers (you do need to get to them afterall, which takes some time - allow an hour per corner of the car).
Next I would look at changing your head unit - future system expansion will be made considerably easier with high quality (and hi-voltage) pre-outs. Furthermore, the quality of signal output is going to be much higher, another factor that decides the ceiling of non-distorted sound; there's no point having a speaker set up capable of producing clear and devestating sound if the signal they're being fed is total garbage.
This also gives the ability to play MP3 cd's or connect an ipod etc etc.
If you were to do this at the same time as adding the sub, you would not have to fit capacitors to the full range speakers - instead, select a headunit that has on-board filters, which do exactly the same job.
This is the setup (high powered head unit, sub and amp, filtered factory speakers) I had in my PD150 and it absolutely rocked, as anyone who heard it will attest.
If, after all that, you still want more volume and clarity, then replacement full range speakers (and then amplifying them) is the route to take. However, as someone that has already been there and done it, I would suggest you improve your signal (ie, swap the headunit and filter the signal to something suited to the speakers) and add a sub to give your sound some depth before you look at messing with the rest of it. I personally believe that amping your standard speakers and doing nothing else will potentially be wasted money - you're going to end up with a screechy and distorted mess of a sound, which is no louder prior to the point it distorts than it is at present.