Security was another concern, particularly with the DSL connection being permanently on the Internet. The solution I devised might not be the best choice for you, but my experiences may help you think through the issues more completely. Quite a variety of NAT or proxy solutions is available on the market. Before settling on a software package that could run on my existing computers, I looked briefly at stand-alone products. In this case you will need to pay for multiple accounts at the ISP and an extra phone line or two.
Some packages can turn standard PC hardware into dedicated firewall routers.
One such product is GNAT Box from Global Technology Associates. IPRoute also lacks some features that initially seemed attractive such as logging and filtering. Ironically, however, the product I ultimately chose was almost as simple. Since I was looking for a relatively inexpensive solution, I wasn't prepared to dedicate an entire PC to function as a gateway. That left me with a number of software utility products.