First, the new Ubuntu Linux kernel will have the RTAI stuff built into the release. It will also have the RT Ethernet drivers included.
Second, we can get a reasonably priced ATmega16 board from Olimex and an ethernet board from the same place. The AVR processors are easily programmed in C and have lots of useful ports. Note: Olimex now has a Cirrus ARM9 board with 100 meg ethernet for 124 Euros.
The ethernet would run at 10Mbits on a dedicated link from the processor. That would give it 10,000 1000 bit frames at full utilization. We would probably run at closer to 1000 frames of 1000 bits each for 10% utilization. Since 10BaseT is full duplex, we could run both directions at that rate.
It should be pretty simple to implement a test bed to try this out. The only open issue in my mind is what the latency might be.