xf86config should ask you, it will be when it asks you if the modelines are ok, giving you a choice of editing the default mode lines for 8 bit, 16 bit and 32 bit...
Either that or you can just edit your XF86Config file in /etc/X11 under the screens section.
Sample File:
Code:
# **********************************************************************
# Screen sections
# **********************************************************************
# Any number of screen sections may be present. Each describes
# the configuration of a single screen. A single specific screen section
# may be specified from the X server command line with the "-screen"
# option.
Section "Screen"
Identifier "Screen 1"
Device "ATI Xpert 98"
Monitor "My Monitor"
DefaultDepth 16
Subsection "Display"
Depth 8
Modes "1024x768" "800x600" "640x480"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 16
Modes "1280x1024" "1152x864" "1024x768"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 24
Modes "1280x1024" "1152x864" "1024x768"
ViewPort 0 0
EndSubsection
EndSection