As for VoodooMem, you have to set it to something below 16. If you set it to 16, you use all the ram on the Voodoo board, and there is no room left for the fast ethernet card. Set it to 15 and you are fine, this way there is 15 MB for video ram, and 1 MB as a buffer for the cards that needs it (like the fast ethernet card).
FYI, the Mediator uses the graphics card ram as a buffer, so without a graphics card installed, a Mediator equipped Amiga can only use a small range of PCI cards. Fast ethernet cards and soundcards will be unusable for instance.
You can test if the ethernet card is useable by querying the device in Miami. If it brings up a MAC adress, the card is suitable and your problem lies elsewhere, probably in not having added a DNS server to the DNS server list under databases.
