The Voodoo3 3000 is the best gfx card for the Mediator due to 1. Voodoo4/5 ain't fully supported by the drivers so you don't get all the screenmodes and 2. the bottleneck in the system prevent you from gaining the extra speed available in the "bigger cards" anyway.
I don't think it'll work with 2 gfx cards but you're welcome to try.
Also with a TVCard you should be able to reroulte Amiga gfx trough it (somehow, I don't know).
Last... 100Mbit Amiga Ethernet roxxorz. 8-)