I forget exactly what the file is called, but it's some log file in /var/log... I believe it's XF86-9.log or something like that. But go to /var/log and type in the console
view ./XF86-9.log (or whatever the file is called... do an ls on the dir first.)
Check for any lines beginning with (EE), those denote errors. You should be getting something like this:
(EE) Failed to initialize the nvidia kernel module!
(EE) Screens found, but none have a usable configuration.
If this is the case, and you're running the latest drivers (the 4192's I believe they are), you must change nvidia to NVdriver. Nvidia will not work on the latest drivers.
If you're using some other version, I don't know what to tell you. I'm having that same problem with the 3123s on my P4 1.6, but they installed fine on my P3 1GHz. Go figure.