You can use the "gtf" program to generate a custom modeline. "gtf xres yres 60 -x" will generate a modeline for 60hz refresh rate.
What video card do you have? If it is nvidia, then download and run the nvidia installer followed by nvidia-xconfig.
The "sudo /sbin/yast2 inst_source" wizard has a community repo section that includes nvidia and ati repos. You would try that as well. If however, you have an older video card that needs a legacy driver, then this may not work.
If a certain resolution is possible, you could run xrandr to change the resolution while in gnome. In kde, there is a krandrtray applet that is much more convenient.
Also, the xorg.conf file has it's own manpage which may come in handy. You might want to search this site or the web for sample xorg.conf files that can show you how they are structured. As of late, the xorg.conf files have been becoming more sparse as the server has been autoconfiguring more things, so you may need to resort to a sample to know where to insert your "Modes" entries, etc.
Sometimes you don't even need modeline entries. Standard resolutions may be handled by by xorg without explicitly providing a modeline. In that case, you just need the resolutions entered in the "Screens" section which might look like this.
Code:
Section "Screen"
DefaultDepth 24
SubSection "Display"
Depth 15
Modes "1280x1024" "1280x960" "1152x864" "1024x768" "800x600"
EndSubSection
SubSection "Display"
Depth 16
Modes "1280x1024" "1280x960" "1152x864" "1024x768" "800x600"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x1024" "1280x960" "1152x864" "1024x768" "800x600"
EndSubSection
SubSection "Display"
Depth 8
Modes "1280x1024" "1280x960" "1152x864" "1024x768" "800x600"
EndSubSection
Device "Device[0]"
Identifier "Screen[0]"
Monitor "Monitor[0]"
EndSection
This are fairly generic entries for my desktops 1024x1024 LCD monitor. I alway use the native resolution.