wheelarches: Outside some butyl (so that's one - two sheets @ 25x30cm)
inside, under the carpet, the acoustistop on top of a layer of butyl would be the ideal way, depends on the space available, my carpet's already tight, but 4mm should be possible.
The two have different functions. The butyl stops the resonance, the acoustistop limits noise transmission.
That's why the butyl makes the doors go "dump" instead of "clong". The Acoustistop is you best friend when we're talking about limiting road noise.
Mine is now actually seriously quiet if the boot's full. Quieter than the wife's 307, I know that, and cruising at 50mph it's perfectly possible to have a quiet conversation. That's with spikes
In an ideal world, you stop the vibrations with mass and elasticity and then something very dense will reduce noise transmission dramatically.
Make sense? If you then fill the space with foam, the remaining noise can't really move anywhere, meaning the insulation is even more effective.
If you wanted to go the whole hog, the doors could get an additional layer of foam behind the cards, which again helps. This thread -
http://www.a2-freun.de/forum/showthread.php?t=24459 - is from a dealer, which explains why it's quite so comprehensive. But it's a very good reference.
Your 4sqm should get you a lot of the way round the car, depending how much you splurge.
Bret