Some resolutions only work if you have enough video adapter memory. Make sure you have at least the minimum DRAM or VRAM to support the desired number of colors at the specified resolution:
Memory requirements for color support at specified resolutions
|(4 bits)||(8 bits)||(15/16 bits)||(24 bits)||(32 bits)|
Refer to your video adapter documentation for additional information on memory requirements.
If you select resolutions of 1024x768 or higher, you may need to determine if your monitor works in interlaced or non-interlaced mode at these resolutions. Consult your monitor's documentation for information regarding supported resolutions.