www.portforward.com

makes life nice and easy, basically just fin your router/modem on that site and then pick any program and substitute 3dgs ports.

I am not entirely sure what ports 3dgs uses but ive heard you basically pick a port between 2300 and 2400 by using -port XXXX in the command line, and also for some unknown reason (well it is; some microsoft port, but thats not the point) open a port thats like 6074(??) not entirely sure was reading a post someone else on the formums a few days ago about it