I downloaded CentOS 5 from the web and successfully install on my desktop computer. The problem is when I changed the moditor type to LCD IBM 9490 (my monitor model number), and restart, I got a blank screen, or something like "problem with display" I don't remember!!
I have to boot linux with rescue mode and change the screen setting file back to the original to get it work (/etc/X11/xx)

As a result, I have to re-install RedHat, and have no problem when setting the monitor to the same type.

Without changing the monitor type, i cannot change the screen resolution
!! Does anyone have a similar problem?

First, if you ever get an error with X not starting you can press ctl+alt+F1 and get a console. From there you can just edit your xorg.con file and restart X. You don't have to reboot or reinstall. I would google and see if you can find the proper settings for your monitor to use with the xorg.conf file.