Ok guys!

Yesterday I finally received all the stuff I ordered to silence my pc.
I've got a :

Q technology 400Watts PSU
1 Zalman CPU cooler for P4
3 pabst case fans, each with their own Zalman Fan mates (to control their RPM / noise level)
1 pabst 40mm x 40mm 12dB fan to replace the one on my Gforce3.
Sound absorption mats to put on the inside of my case.
MacPower Digidoc temperature and fan controller (to monitor the whole thing when in use)

Total cost : about $350

Findings :

The sound level has definitely decreased, but not really to the level I wanted it to. I found out the biggest noise pollutors are actually the two 7200 RPM harddrives AND THE NEW PSU! Concerning the harddisks : currently I have no solution to fix this. The only option to fix this I know of (the silentdrive enclosure) will not work for me since that is a solution for 5400Rpm drives ONLY.
The new PSU however is a different matter. I put my old PSU back (a chieftec 340 Watts), and the difference in noise level was very noticable. Needless to say that new "silent" PSU will be returned on monday.

The Zalman CPU cooler, though I had my doubts about its effectiveness at first, really turned out to be the big surprise of the lot. While the stock Intel CPU cooler cooled my CPU to a decent 36°C, the Zalman cooled it to
32°C !! (in quiet mode, so this is with the supportive fan at only half speed - I think 30°C should be obtainable with full speed, but I don't see the point). Actually I believe the temperature would have increased, wile in fact it decreased. Quite a nice surprise!

Would I do it again ?
Mmm, hard to say. While the noise level had definitely dropped, it has not dropped to the amount I thought it would. Mostly this is because of the HD's though, so if I could find a solution for that, chances are I will look entirely different upon this.