I know that when i installed my Mandrake it asked me during the install. Otherwise I know that in mandrake its in the hardware section of the gnome configuration under Display, I'd look under such a place in your version. Personally i wouldn't spend to much time in gnome, i used to use that instead of the command prompt but have found my self moving more and more towards the shell because of the power that it possesses, and its remote ability to be accessed from any computer on he net.
Regards,
Skull
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Trust me, if i started killing people, there'd None of you left