You need to edit your XF86Config file...most likely to bring the resolution down. You might not have the drivers installed for your specific video card (generic linux ones install by default) and your monitor might not be specified either. This means that your monitor might not or video card might not support higher resolutions. If during the install or config of your XF86Config you specified a higher resolution than 800X600 then you xserver will most likely crash if the higher res isn't supported. Pump this resolution down by going into your CF86Config and editing it manually. For more information: Try the XF86Config Manual page