Option "ModeValidation" "NoEdidDFPMaxSizeCheck"
I'm not familiar with that option. Supposed to give you the maximum resolution?
I just use the following in my Modes line
Modes "1024x768" "800x600" "640x480"
This allows me to use these any of these three modes which are valid for my monitor. I suppose you could check the documentation for your particular monitor and enter every valid mode that the documentation lists. That should work.
If I just wanted to use one mode, I could just enter
which would only allow me to use 1 resolution: 1024x768.
Use <ctl + alt + - > or < ctl + alt + + > to switch between other entered resolutions on the fly.