I know this works in ubuntu
I think this may work Mandriva as well:
Open a terminal and type:
sudo nano /etc/X11/xorg.conf
Press ctrl+w and search "Modes"
You should see a line similar to:
Modes "1024x768" "800x600" "640x480"
The first resolution "1024x768" is default
So if you wanted to make 1024x1028 the default resolution, you change it to:
Modes "1024x1028" "1024x768" "800x600" "640x480"
Then press ctrl+o to save
And reboot