tyggna, you advised me to change the monitor's driver. I did't know what you mean so I changed my graphic card's driver. It was set to nvidia - now is nv. That helped. When I change the value of VertRefresh it really works. I checked it in the monitor's onboard configuration.
lazlow, you're absolutely right. It was mistake. I wonder why I thought that 1240x768 was a proper mode?
Do you know why the previous driver doesn't follow the options set in xorg.conf? I typed nvidia-installer --version and I got
nvidia-installer: version 1.0.7 (buildmeister@builder3) Wed Jun 13 18:39:37 PDT 2007
The NVIDIA Software Installer for Unix/Linux.
This program is used to install and upgrade The NVIDIA Accelerated Graphics Driver Set for Linux-x86.
Copyright (C) 2003 NVIDIA Corporation.
This driver is quite new. I remeber that I compiled it a few months ago after I downloaded it from www.nvidia.com