edit your /etc/X11/XF86Config or /etc/X11/xorg.conf (whatever your system uses).
You will find in there something like that:
Code:
Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1024x768" "800x600" "640x480"
EndSubSection
EndSection
change default depth to 16
u cant also do that from X-windows i think, under System Settings->display or something like that.