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The 360 doesn't output 1920x1200 as far as I know. There's is the 1920x1080 setting in your 360 dashboard video settings that you must change when changing between cables. Then your monitor will fit this resolution appropriately. Also, try turning your sharpness to 30 in your 2407 settings, this gets rid of a lot of problems.
My setup:
VGA Cable (Microsoft)
1920x1080
2407 sharpness to 30
XBL 'Fall update'
Also, are you running on xbox live? If not, you may not have the latest xbox 360 dashboard update. This would prevent you from getting some of the higher resolutions on vga.