For Mandrake, you didn't say if you are seeing your screen clearly.
I will assume that you are, and
if you are using gnome go to the main menu and choose configuration.
you can make the change to the resolution there. I use 800x600 at 16 because I have it installed on an older system and that helps in speed too.
For some reason, my system won't run the KDE so I can't guide you with that.
Based on this info you should be able to find the configuration in KDE as well.
Then just click apply and it should be okay.