Well, two or more ethernet ports are useful to one or more of:
- don't want to buy a gigabit switch and just want to connect one machine GbE, the rest 100 MBit over a cheap switch
- one interface to internet, one to LAN
- have your wireless - connected hosts and the hosts connected over cable in your LAN on different interfaces to firewall them (to make people using your wireless net not be able to snoop around the cable part)
- same, but with two wireless network when you have one less and one more secure
None of the require the second card to be GbE.
Two actual GbE are only useful if you are actually pumping data to multiple hosts simulnateous which you probably don''t at home.
Unless you have PCI-X or PCIe GbE cards the normal PCI bandwith is not sufficient to sustain two GbE at full speed at the same time, not to mention what happens to disk activity at the same time (can you say traffic jam?).
Note that a number of PCI-X and PCIe board have onboard LAN which is just normal PCI.