maan i hate routers! it should be possible to set them up right so they let both servers throw tho. just that i hate those slow updating in the modems and 10 000 tries before it works (if it does because the cheap crap modem decides not to)
anyway try this place:
http://portforward.com/by the way its much easier to the computer you have the servers on act as an router, then there will be no more router problems and its an inbuilt function in XP (maybe just xp professional not sure) oh and you need two network cards in the server computer to and a switch to the rest of the computers ( or you can use the inbuilt switch in the router if you set it up right )