Mira en las secciones "Monitor"y "Screen". Aqui te pego el mio con una MDK:
Section "Monitor"
Identifier "monitor1"
VendorName "Samsung"
ModelName "Samsung SyncMaster 17GL"
HorizSync 30-65
VertRefresh 50-100
# Sony Vaio C1(X,XS,VE,VN)?
# 1024x480 @ 85.6 Hz, 48 kHz hsync
ModeLine "1024x480" 65.00 1024 1032 1176 1344 480 488 494 563 -hsync -vsync
# TV fullscreen mode or DVD fullscreen output.
# 768x576 @ 79 Hz, 50 kHz hsync
ModeLine "768x576" 50.00 768 832 846 1000 576 590 595 630
# 768x576 @ 100 Hz, 61.6 kHz hsync
ModeLine "768x576" 63.07 768 800 960 1024 576 578 590 616
EndSection
Section "Screen"
Identifier "screen1"
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
EndSection
Fijate que es en "Screen" donde tengo puesto que se vea a 1024x768
Un saludo