Well, you'd get GigE between the computers that supported it but not out to the internet. Depending on how many filed you transfer between the machines, it may be worth it.
When I went to gigibit from 100mbit it wasn't much faster for computer to computer xfers. You start to run into the bandwidth limits of the PCI bus, especially if your hd controller is on the PCI bus. If your nic is integrated in your chipset or its on pci express this might be different.
