I have a voodoo 3 installed with X 4.2.0
The Glide libraries can be found on the extras cd. Or in the contrib directory.
From an Australian mirror
ftp://mirror.aarnet.edu.au/pub/slackware/slackware-8.0/contrib/3dfx/
You may want to look in a closer mirror.
In the kernel 2.4.18 or a similar version, look under Character Devices and there is a DRM DRI section there is a 3Dfx Option, It works for the Voodoo 3 and you don't need another module. I don't know about the Voodoo 5 as my computer is not that 1337
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I hope that helps, personally I have never tried OpenGL since I upgraded to X 4