What ports do I need to open, and how do I do it?

I have a Vigor 2820, but I am guessing this will be very similar for you.

Assign your xBox a static IP Address so that you can forward ports to it.

On the routers web interface (under Nat and then Open ports on the 2820) you need to setup a new open port.

Enter the IP address that you assigned to the xBox and then add the following ports

1) TCP port 80
2) UDP port 88
3) TCP/UDP port 3074
4) TCP/UDP port 53

I hope this helps.
Dave.