If the VooDoo card has its own drivers, then do what I do for my Picasso card. I understand that a VooDoo will run with the Picasso96 drivers(?) Anyway, go to the Image Format menu and select the Picasso/VooDoo screen format required (They're all 8-bit I'm afraid). Make sure you activate the 'RTG' button. Click Proceed.
The sceen should now be in the selected Picasso/VooDoo mode.
Now go to the Settings menu and click the Proceed button. This saves the startup prefs you just set up.
Cheers,
JaX