I had about the same problem. Right after the installation I had a running xserver only with wrong monitor settings (some VESA with only 60 KHz). When I try to change this (no matter what use: Sax2 or YaST or XF86config), I get the described black sreen.
I found out, that I can restore at least the last running xserver by first backing up the /etc/X11/XF86config before playing around with SaX2 or YaST.
Tried to update to the latest NVIDIA driver; no help.
At the moment I'm trying the Suse Helpdesk to figure out the problem; when I find something; I'll tell your.