It's not 1U and it's not real world, but if memory serves me I was getting throughput around 760Mbps full duplex on iperf with a Sempron 2800+ (1.6GHz), ASUS M2NBP-VM CSM (onboard nvidia GBE), Intel Pro 1000 PT pci-e. The cpu was maxed during the test. That was through a pair of bridged interfaces too. I don't remember the throughput using NAT, but obviously it was lower.
Substitute the 1000 PT with a 1000 GT pci card and my numbers were lower, around 400Mbps, presumably because of pci bus saturation, as the cpu was ~70%. Interestingly, device polling made no noticeable difference on throughput in my tests.
May I take advantage of this moment to plug my comment and poll on hardware benchmarking? I think I'll link to it every time I answer a question like this. Please check it out.
http://forum.m0n0.ch/index.php/topic,875.0.htmldb