OK...
First off, with DSL, the bandwidth is all yours. Whether you have one PC or 50 PCs, the bandwidth is yours to do with as you please. Unlike cable, you're not screwing over other subscribers by stealing their bandwidth. Sharing the connection splits the bandwidth into smaller chunks, it does not draw more.
The one computer rule comes from the number of IP addresses they want to hand out - Only one, unless you're paying for more. Whether that IP address goes to a Windows 98 machine, a "broadband router," or a linux firewall, they couldn't care less.
NAT allows you to run multiple systems behind one external IP address, there is no way to tell how many machines you have behind it. All the ISP sees is one machine, & one IP address. You have therefore met their requirements. On the other side of the router you're running private non-routable addresses (192.168's, 10.'s, etc.). Those are not valid public IP addresses, your ISP does not and cannot see those. Nor do they care about them.
Bottom line is you are not committing a crime, & you are not violating your terms of service agreement.
You're fine.
Viper GTS