You have to set kernel according to advice up and after check your x86config in /etc/X11 (slackware) this lines:
Section "Device"
Identifier "Voodoo5" # type cards
VendorName "3dfx" # manufacturer
Driver "tdfx" # driver for cards with 3dfx chip
EndSection
and
Section "Screen"
Identifier "Screen 1"
Device "Voodoo5" # devices
Monitor "My monitor"
this element's must be equals ==> Section "Device" --> Identifier
"Voodoo5" with Section "Screen" --> Device
"Voodoo5"
this is my config
http://rabbit.phorum.cz/config