[ALUG] VPN

Hi , I want to setuop a VPN but I don't know how to do this: So we have: mikes PCs on internal network --> mike's main pc --> second network card --> cable modem --> internet --> my ADSL modem --> my linux firewall. My modem has an external address but uses NAT to convert to internal address before arriving at my linux firewall. I can set up port forwarding. some ideas ? Cheers Vasko