Here is a snippit of my Xfree86-4.conf file in /etc/X11/. Notice that the DefaultDepth is 24 and in the Subsection "Display" Depth 24, it starts with "1280x1024", which is the default that the computer will try first:
Section "Screen"
Identifier "Default Screen"
Device "Generic Video Card"
Monitor "Generic Monitor"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1280x1024" "1280x960" "1280x854" "1280x800" "1280x768" "1200x800" "1152x864" "1152x768" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1280x1024" "1280x960" "1280x854" "1280x800" "1280x768" "1200x800" "1152x864" "1152x768" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 8
Modes "1280x1024" "1280x960" "1280x854" "1280x800" "1280x768" "1200x800" "1152x864" "1152x768" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1280x1024" "1280x960" "1280x854" "1280x800" "1280x768" "1200x800" "1152x864" "1152x768" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1280x1024" "1280x960" "1280x854" "1280x800" "1280x768" "1200x800" "1152x864" "1152x768" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x1024" "1280x960" "1280x854" "1280x800" "1280x768" "1200x800" "1152x864" "1152x768" "1024x768" "800x600" "640x480"
EndSubSection
EndSection
Section "ServerLayout"
Identifier "Default Layout"
Screen "Default Screen"
InputDevice "Generic Keyboard"
InputDevice "Configured Mouse"
EndSection