Lots of info on Mac graphics cards (including the 3dfx Voodoo 1 thru 5) here http://www.xlr8yourmac.com/video.html

One quick check - if there's a male and female 15 pin connector on the card, then it's either a Voodoo 1 or Voodoo 2 card. In this case, no flashing is needed - PC and Macs use the same card.