Thanks to everybody who replied, I got X working :) I just didn't upgrade all the XFree86 packages - that was kinda stupid, oh well...
My desktop is 'working' now - but it's totally messed up! When it starts I get this message: "You are not running a GNOME Compilant Window Manager. GNOME support by the winodw manager is strongly recommended for proper Desk Guide operation." I did an apt-get install sawmill-gnome - but I still get this! There are also no buttons ontop of every window to maximize, close or minimize it. I was using sawmill before I tried to upgrade some packages in order to get Gimp 1.2 running. And now I'm here having a totally messed up desktop. Any ideas? - any help appreciated! TIA Philipp