My recommendation for an inexpensive hardware firewall device for small business networks is the Smoothwall firewall.  This is a Linux open source product that will run on older hardware.  Installation and configuration is relatively easy and requires minimal maintenance once placed in operation.  If you are interested in discussing a firewall installation for your business, please contact me for further information.

A hardware firewall is a dedicated computer that is placed between your local area network (LAN) and the Internet.  All traffic entering and leaving your LAN is routed through the firewall.  The computers on your LAN use private IP addresses and are effectively hidden from view from the Internet, increasing the security of your network.  

Besides increasing security, the firewall acts as a router so you can have from one to hundreds of computers on your LAN sharing one Internet connection with a single public IP address.  

The Smoothwall firewall can be easily configured with up to four separate network zones:
1.  Red - The internet connection (untrusted) with the public IP address
2.  Green - The local network (trusted zone) using private IP addresses and can have multiple computers
3.  Orange - An optional "DMZ" connection for servers (Web or email servers) that need external Internet connections
4.  Purple - An optional zone to provide Internet access to publicly shared wireless access points.

Smoothwall resources:

The software downloads and additional information are available at:  http://www.smoothwall.org  
Ad-Zapper and other Smoothwall mods:  http://martybugs.net/smoothwall
Customizing Firewall Rules:  http://senecacomputer.com/editrules.htm
Block firewall hits from China and/or Korea:  How to block Smoothwall firewall hits from China and Korea
URL Filter add-on for Smoothwall:  http://urlfilter.net
Firewall open port test:  https://www.grc.com/x/ne.dll?bh0bkyd2
A few shell scripts for Smoothwall firewalls are on the Linux Scripts Page.
Editing Smoothwall/Linux:  http://www.angrypeanut.com/geek/editing-smoothwall-and-linux/