I think a dual nic is the best solution, but it's going to be difficult, if not impossible to do if you don't have a dedicated IP address. If you don't have dedicated IP, the next option is this software:

http://www.winproxy.com/products/winproxy.asp

which might work with a dynamic IP. You can try it for free, and pay like $60 if you want to keep it. It will set up your entry point computer as a proxy server that then provides gateway services to other computers on the network.