how to change XFCE4 screen resolution

[root]

hi everybody, 
      Recently, I use Fedora 11 and installed XFCE 4.6.1 as my priority
choice of GUI and also installed GNOME. The screen resolution, however, 
could not change after rebooted. The screen changed all right on the
process of XFCE-session until reboot. I figured out display.xml within
the $(HOME)/.config folder, the resolution value has changed. When I
rebooted, the display resolution restore. Any suggestion I will
appreciate.