Is there any advantage point of having a Gigabit Ethernet if the PCI bus connecting NIC to Memory is a lot slower than that say just 80 MBps? In other words, I am trying to conclude that it makes sense to use high speed network only if the bus connecting the NIC to RAM is fast enough, else its of no use. Correct me if I am wrong.
This was a problem when 1 Gb NICs first came out. Servers could only get about 400 Mb of throughput on a 1 Gb network because the server bus wasn't fast enough to get 1 Gb. It was still four times better than 100 Mb, even if it couldn't use the full bandwidth of the NIC. The bus used in PCs and servers has caught up with 1 Gb NICs, and there are newer bus speeds popping up every few years.
It would be wrong to say it is of no use if the bus is too slow to take full advantage of the NIC speed since it can still be faster than a slower NIC.