I am using an X-Surf 3cc ethernet card with my Amiga 4000. It's painless to set up and it works a snap with Genesis.
It's an old-style Realtek PC ISA card welded and riveted onto a bridgeboard, and you get a couple clock ports and a simple IDE interface thrown in for free! :-D
It costs roughly 70 GBP from Vesalia or Amigakit I think, but it's been worth every penny to me - makes shifting stuff to and from the Amiga much easier and saves on the pain of hunting down an Ariadne or similar. :-D