Ok, I'm finally up and running (almost) thanks to
Sidux which is an installable live cd based on debian unstable and which uses a 2.6.20 kernel.
Installation was pretty painless except for the installation of my Geforce 7900GT which refused to work with the live cd - I couldn't perform the installation.
So I ha d to plug another card into the box, perform the installation and then swap the cards to use the new 7900GT.
And now I can't change the refresh rate of the monitor at all, it's stuck at 50Hz and whatever I do I can't get the 85Hz I always had until now with plain debian.
Any ideas? All I succeeded in doing until now is a black screen when I change anything in xorg.conf ... and 50Hz is just not good for my eyes, I can feel that!