I remember having the same problem when I tried to use a network card with my Mediator SX, but I found some instructions on the net which told me to insert the network card in the first slot, and the Voodoo in the last. I did this and got things working.
Currently, this is how my Mediator is set up:
Slot 1: Network card
Slot 2: Soundblaster
Slot 3: TV card
Slot 4: Voodoo
