网上说在/etc/X11/xorg.conf中加Virtual，再删除其他分辨率
Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
Virtual 1280 800
Modes "1280x800"
EndSubSection
EndSection
一开始找不到xorg.conf文件，
yum install system-config-display 以后就好了
至于装了gdm也装kdm，要切换，就要装system-switch-displaymanager-gnome，system-switch-displaymanager