I am a Mandrake user so I don't know if I can help or not but........
Make shure you have the correct kernel source for your running kernel installed. Then download the nforce source rpm and follow the install instructions on nvidias site.
As far as the blackness have you tried to edit your xconfig file and set it back to where it was before you changed monitor types?
There are seldom technogical answeres for behavioral problems.