Revisa tu /etc/X11/XF86Config, y verás algo como esto:
Section "Screen"
Driver "svga"
Device "device1"
Monitor "monitor1"
DefaultColorDepth 24
Subsection "Display"
Depth 8
Modes "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
Depth 15
Modes "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
Depth 16
Modes "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
Depth 24
Modes "1024x768" "800x600" "640x480"
EndSubsection
Subsection "Display"
Depth 32
Modes "1024x768" "800x600" "640x480"
EndSubsection
EndSection
Observa este detalle:
[...]
DefaultColorDepth 24
[...]
Subsection "Display"
Depth 24
[...]
En esa subsección mete esto:
Subsection "Display"
Depth 24
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubsection
Reinicia las X y vuelve a entrar.
Un saludo