How was it possible for you to install xorg 7.4~5? When I look at the dependencies then I see that xserver-xorg-input-wacom 0.8.1.6-1+ provides xserver-xorg-input-2.1 and xserver-xorg-input-wacom 0.8.1.6-1 provides xserver-xorg-input-2. Both conflicts with xserver-xorg-core 2:1.5.99.901-2. It is nearly the same with all video drivers which provide xserver-xorg-video-4 which conflicts with xserver-xorg-core. Seems that the xorg packages are more or less broken on debian experimental (amd64)