Right but that would be just one PCI NIC moving data to or from the hist machine. If you have two NICs in a routing configuration on the same bus I would expect the throughput to be half that or less.
What I could never pin down is whether it should actually be a quarter of the maximum bus speed due to having to send any return data.
Looking back at my own results the box I used could only manage 200-250Mbps between two the PCI NICs. It seemed like it was a quarter bus speed. :-\
Without some form of logic analyser it is probably difficult to determine average or typical DMA burst lengths on the bus. And the amount of "return data" including TCP ACKs is highly application dependent.
I think in many cases a bit of analysis is useful to determine if a configuration is going to "work", rather than try to guess to the nearest 10Mbps the throughput that might be able to be achieved. In another topic still being discussed, the author is complaining of not getting Gigabit speeds between two interfaces. From additional information posted it looks as if the pfSense box is one of the generation where all the IO devices were hung on a single PCI bus or an ISA bus. The analysis shows there is no way to sustain 1Gbps between two NICs on the same PCI bus, let alone a bus also shared with disk controllers, ISA bridge etc.
