You can use Vista's super annoying new Network and Sharing Centre to find out which of the interfaces is the one actually connected to the internet, it'll be under the network that has both
Local and Internet under Access:
Click View Status and the Details tab to find out the Default Gateway
If you're plugged directly into your Motorolla Modem, you will not need to perform any port forwarding because all packets will be forwarded to your PC as it is directly connected. You'll just need to open the port in your PC's firewall if you have one. Your modem probably doesn't have any sort of web of web interface apart from maybe some diagnotic tools loaded by your ISP.
If however, your modem is connected to a router, you will need to log-in to your router to forward the ports and allow it in the router's firewall. Most routers use 192.168.0.1 or 192.168.1.1 as their default address so unless you've changed it, try them.
If you're in the UK, Orange Liveboxes use 192.168.1.1 by default and just to be annoyingly unique, BT's Home Hubs use 192.168.1.254.