...but the hdds are mostly the problem.. I dunno how much i'll gain by going fanless...
To test the noise produced by your HDDs then just power them down. To do this quickly and easily without breaking anything then simply put your computer in suspend mode (through the "turn off computer" dialog in XP) - this should keep all fans running but power down your hard disks.
Then have a good listen to see the difference.
Pressing a key or moving the mouse should wake up the disks and OS.