Well, since you are using a 100 MBit ethernet card, you will not get it to work before you have a graphics card installed, as the ethernet card uses the ram on a graphics card as a buffer or something like that. Furthermore, the device driver for your ethernet card would be called FastEthernet.device, the MediatorNET.device is only for 10 MBit cards.
Either get a graphics card or get a 10 MBit ethernet card. Take the first option, you will not regret it, as you will then be able to have your Workbench in 16, 24 or even 32 bit color in resulutions of 800 x 600, 1024 x 768 or higher, and it makes browsing much more pleasant. Furthermore, the graphics card is faster than AGA anyway.