WEll I didn't say they were "official" 3dfx beta's...I just said grab these and try them. They worked very well for me. Boosted my framerate in UT from an average of 35-40 to over 50. But like you said what might work for one won't work for another.
Though those x3dfx drivers don't seem to work for any win 2k users.
Oddly enough I do seem to remember 3dfx was set to release a set of win 2k beta drivers...which was just before they decided to tuck tail and run.