If your graphics card is nvidia powered, look at the man pages for nvidia-settings and nvidia-xconfig. You can configure your card from the command line by running /usr/bin/nvidia-xconfig.
You can add the resolution to your Xorg config file with
You can replace the resolutions in your Xorg config file by giving nvidia-xconfig a list of resolutions
nvidia-xconfig --mode-list=1600x1200 1280x1024
When done, use the command startx, startkde, or init 5 to get to graphics mode.