OCModShop posted part 3 of their Linux Firewall series

As mentioned in the previous segment, you can create a professional-level firewall using old hardware that you would otherwise throw away. Some people may choose to purchase new dedicated hardware, which can have several advantages. Either way, you can create a hardware firewall costs significantly less than the $1000-$3000 that professional hardware devices can cost.

Linux Firewall Part 3: Selecting Your Hardware