You should indicate what monitor you have. Also post the relevent sections of your xorg.conf file.
Check the most recent Xorg log in /var/log/. Errors are on lines preceded with (EE).
One thing that can cause a problem is if your vertical and horizontal rates are off for the monitor in the monitor section.
Also try running (as root) the nvidia-xconfig script. That may fix up your device section in xorg.conf.
See if you have other resolutions in your xorg.conf "Screen" section. If you do, and the refresh rates look OK, consider using the gtf program to generate the modelines to use.
gtf 1024 768 60 -x
# 1024x768 @ 60.00 Hz (GTF) hsync: 47.70 kHz; pclk: 64.11 MHz
Modeline "1024x768_60.00" 64.11 1024 1080 1184 1344 768 769 772 795 -HSync +Vsync
Copy the output from the terminal and past it into the xorg.conf file; in either the Monitor section, or if you xorg.conf has one in a "Modelines" section; add a "Use Modelines" line referencing the Modelines section in the Monitor section. ( It probably already does, and you may have an empty Modelines section as well ) Change the mode line label for "1024x768_60.00" to "1024x768_60".
The xorg.conf file has it's own manpage that you can reference while editing.