Strickly evaluating the two cards you mentioned, Voodoo 5 & GeForce 2 MX, the Voodoo 5 will win hands down.
However, since 3dfx is no longer in business, if you purchase a Voodoo 5, there will be no warranty and no driver updates.
The GeForce 2 MX card is a good one, if you don't plan on running games like Quake 3 above an 800 x 600 resolution. You can find GeForce 2 GTS cards for less than $200.00. Another good line of video cards are the Radeons from ATI. I believe the Radeon LE is priced about the same as the GeForce 2 MX and is equal in performance. Finally, Hercules is producing a card based on the Kyro chip. The review at Anand's showed in certain situations it could out perform GeForce 2 Ultra card. The Hercules Kyro card will list for about $150.00