I've been doing some tests with iperf to measure the network throughput between 2 servers (connected back-to-back thru a crossover cable) and I only see 180 to 200 Mbps. I tried different cables and they're all CAT6. As you may know, iperf generates packets on the fly so disk I/O is not involved here...
The two servers are running plain-vanilla Windows 2003 with the correct and latest network driver for the NICs. I know there's some protocol overhead involved but I don't expect this to take more than 20% of the bandwidth.
Is it realistic to aim for 80% utilization? That is, to get around 800Mbps on a 1Gb link? Have you actually seen this on Windows?