[CentOS] 1U firewall hardware

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.