I think there's some option to set a certain machine as DMZ or something in some routers (dunno if it applies to these routers), which will let you keep the internal IP dynamic and still have the ports forwarded to the correct machine.

Adding to Unne's post, I would not forward 6667 either, since that's the default for IRC/chat servers, and I don't see why a games require that open. You shouldn't be getting any traffic on that port (in general) unless you're knowingly running such a server.