You need to remove the # before Modeline (or the whole line will be read as a comment).
Also you need a correct Screen section, something like :
Code:
Section "Screen"
Identifier "your_screen_ident"
Device "your_device_ident"
Monitor "Monitor0"
DefaultColorDepth 24
Subsection "Display"
Depth 24
Modes "1024x768"
EndSubsection
EndSection