I have a home gigabit network via a DGL-4300 router, which does not support jumbo frames. I've measured OK throughput between the computers, even when using gigabit via the PCI bus (whether on adapter cards or built into the motherboard). By "OK", I mean as low as 615 Mbps typical, and perhaps even as low as 350 Mbps between two old/slow computers / gigabit network cards.
On properly designed built-in gigabit ethernet, I get close to full 1 Gbps bandwidth as measured via network benchmarking tools.
That said, I find it very hard to get anything like full bandwidth usage when transferring files, despite using RAID arrays on both ends. While I'm usually able to often break the 100 Mbps (10 MB/s) fast eathernet bandwith easily, justifying the move to gigabit ethernet, I've yet to seriously use the additional bandwidth in file transfers, even when using really large (e.g. 100 GB backup file, etc.). And I see little CPU utilization at these times.
I think that disk write speeds are a big part of the bottleneck at the moment for me, and I have no idea why I'm getting crummy disk write performance on a 3-disk RAID-0 array. Something to figure out..
So at this point, I find that gigabit ethernet bandwidth without jumbo frames is not close to being fully utilized, and the dream of 100 MB/s file transfers (or anything close to that) is still just a pipe dream for me. Yet it's definitely better than 100 Mbps, and certainly worth trying / doing right, as the devices are not expensive and even common.