I wrote an article about the subject of getting beyond the limits of gigabit network file transfers. My solution is to use multiple gigabit network cards and use Linux interface bonding to create virtual 2 gigabit network interfaces. The solution is to use mode 0 or round robin bonding. I do not use a switch although this also works fine. Instead, I just connected two cabled between the two machines.
In my original article, I couldn't get pas 150 MB/s file transfer speeds so the results weren't that great. However, these poor results were due to hardware compatibility issues. Although the on board network card worked fine, the intel e1000e card in the PCIe slot didn't perform well. I replaced it with a HP Broadcom card and everything is working smooth now.
With two gigabit network cards bonded together I can achieve 220 MB/s through a single file transfer over NFS.
It would be interesting if a quad port server adapter would be able to achieve 440 MB/s network speeds, but I don't have the equipment to test this.