Not 100% sure about mandrake, but there's probably an option for that in mandrake control center. The old-fashioned (old in linux means console) way is to open an terminal window (DOS promptish thing), type 'su' and the root password when prompted (this changes you to root if you aren't already...you shouldn't use root anyway), then type 'xf86cfg -textmode'
That should bring you to the xfree86 setup screen, and you can alter your settings there. It helps to have your monitor refresh rates handy, then you can enter the custom values for your screen. Then, kill your x-windows if you know how (if not, learn the 'ps' command, and the 'kill' command) then restart X, and if you don't know how yet, you can also just reboot.
If you set something wrong, and your screen goes haywire, hit ctrl-alt-backspace to stop X from trying to load. Then ask here again, if you have a computer (or windows on another partition) to ask from.