Correcting the display resolution

I have an issue with the display resolution under Xfce. The monitor can handle 1024x768, but the screen display is 800x600. The control panel does not allow me to change this.
I also found the file .config/xfce4/xfconf/xfce-perchannel-xml/displays.xml in which I have made changes, but after a restart these changes get lost.
What makes this issue even stranger is that a previous installation of the exact same distribution (FreeBSD 8.1), did yield a resolution of 1024x768 from the start.

UPDATE: Something else that has puzzled me is why the refresh rate is at 61Hz, as opposed to a choice of 60Hz and 75Hz, with no option to change this.