To get Moderate NAT, you basically only have one choice. As such, you basically have two options for achieving Open NAT: 1) Create a DMZ (Apple calls this a Default Host) for the XBox or 2) Use a non-Apple router that supports UPnP. This is primarily due to the fact that the Apple routers do not support UPnP.
What it comes down to is that although you can get an Internet connection for the Xbox with the AirPorts, you may not get the necessary NAT setting (Moderate or Open) for the Xbox Live! on-line game that you want to play. Start by determining if your network hardware is supported for Xbox Live! You should find that the Apple base stations are not.
