You can use something like this Atom 525 dual core motherboard:
http://www.jetwaycomputer.com/NF99.html
Or this Atom C550 dual core board:
http://www.jetwaycomputer.com/NC9C.html
With the AD3INLAN-G daughterboard:
http://www.jetwaycomputer.com/Daughter_Board.html
This will give you 5 Gigabit Ethernet ports (2 on PCIe and 3 on PCI) and a free PCI slot on which you can put up to 4 more. Of course it all depends on the needed concurrent traffic.