Itojun requested me, a while back, when I begged for some information,
to report back or summarise my findings about such beasts.

I eventually managed to get hold of a D-Link DFE-570TX 4-port Ethernet
adapter (D-Link label it "Quad Channel Server Card") which exceeds my
requirements considerably (10BaseT would have sufficed, 100BaseTX -
the X is for full-duplex, is it not? - was overkill).

Other than two kernel messages:

de3: abnormal interrupt: transmit underflow (raising TX threshold to 128|512)

and

de3: abnormal interrupt: transmit underflow (raising TX threshold to 160|1024)

it seems to operate fabulously.  I have only experimented with a
single port (some things just don't seem to fall into place by
themselves), but so far I have no complaints.  I'm using it to
transfer large volumes across an NFS-mount, and it seems quite
passable, even performance wise.  Maybe not screamingly fast, but
quite acceptable.

As far as the other offerings that were suggested, I would think
they'll all be better than my expectations, but cost and availability
in South Africa did not rate very highly.

I should think that the TX threshold for the D-Link may need sensible
adjustment on start-up, I am not well-versed in these matters to
search out the details, but other than that, I think anyone who needs
to span numerous subnets could go this route successfully.

++L