took a lot of googling and tinkering but i managed to get Diablo 2 to run in 3dfx glide (despite Blizzard's best efforts to lock in in direct 3D). it's like a different game - the game i used to play back when i had a voodoo 2. the colors are more vibrant, the dark is darker, the effects are more detailed, the animations smoother, the lighting quality could hold up to a modern game and despite it being locked at 25fps in both cases, with glide i couldn't tell that it was not 60fps if i didn't know it.
for anyone interested, you have to use a glide emulator like dgvoodoo and make a shortcut for the game that has as target file "C:\Program Files (x86)\Diablo II\Diablo II.exe" -3dfx