I was able to fix things up for my Dell 2407 using /usr/bin/system-config-display, which appears in my kde menu at Administration/Display. You'll need to enter the root password to use it.
* Select the Hardware tab.
* Click on "Configure" in the "Monitor Type" section.
* Browse down to "Dell", expand that entry (by clicking on the > symbol).
* Select the line representing your 15" monitor.
* Click the "OK" button.
This changes section "Monitor" in /etc/X11/xorg.conf, and should fix up the HorizSync and VertRefresh settings in that section. As I recall, you need to stop and restart X to force the new setting to take effect.