The best card there is in the whole amiga history is the Deneb: gives your fast USB, ethernet through an ethernet-USB dongle, and AHI audio through an USB audio dongle if your are Zorro III, not to mention the flashROM.
All that on an half-sized zorro board that won't even waste an inline ISA slot.
I'm glad I saw this... question on the USB Ethernet Dongle that you mentioned...
I was planning on getting a DENEB and an Ethernet card for my A3K but I actually have a NetGear Ethernet Dongle sititng in my garage. Where would I find a list of supported dongles, if one exists?