I have recently come across a way to FLASH the BIOS on my Voodoo 5 5500.
My original BIOS was ver 1.06
I succesfully flashed it to the latest(?) ver 1.18
I seemed to pick up about 6 FPS or so ont the Aquarium. Not much, from 128 FPS to 134 FPS.
No other effects that I know of yet.
I downloaded ALL of the files this guy had for the Voodoo series
http://homepage.ntlworld.com/g.stanton/Html/main.htm
I don't know if this will fix the windows XP problem, but he says his driver works fine in XP! Both D3D and GLide!
Good luck with them!