Instead of restarting your computer, you could restart the X server (ie. login again) this would save you a lot of time.
The X configuration is stored in /etc/X11/XF86Config, or /etc/X11/XF86Config-4.
Try to look through the different sections. There is a Monitor section which defines your monitor, a Device section which defines your graphics adapter, and a screen section which combines these components (into one screen..)
The screen has several subsections. Usually it looks like this:
Modes "1024x768" "800x600" "640x480"
this tells X that it can use "1024x768", "800x600", and "640x480" if we have 16 bit colors.
play with these settings, the highest resolution will always be used in flavor of the others.