To solve problems with hardware 3D acceleration, I had to run "nvidia-xconfig" manually, as root (I suppose there is a similar script for ATI or other cards), after reinstalling nvidia-drivers.
Reinstalling artwork and gtk+ solved the problems with icons, emerald, etc.
The "KDM fatal error" seems to appear because it tries to login with "sabayonuser" account (it tries to do that on every following login!).
When upgrading, this user doesn't exist.
Are there any operations that should be done after this auto-login that I have to do manually to fix the situation?