Jim,
A 350MHz FreeBSD box can easily handle three interfaces running 30~40Mb/s both directions on 100BaseT/FD interfaces. No network performance issues seen running consortium gated and gtting a full routeing table.
How well have you tested this across the packet size spectrum? Yes it seems to work for best efforts bursty traffic but be a bit stringent about the different packet sizes and you find that various flows get different levels of "quality". As an internet router this works ok with ethernet.
I would be very reluctant to try it with ATM interfaces of custom hardware for hisgh speed serial interfaces.
I know someone who got an ATM card to work and for a single PVC upto around 70M it was fine, it only fell apart when you started to mess around with the packet sizes and we believe that to be more of an issue with the PC than the ATM card or driver. Neil.