Several things could be hanging you up:
First, the 2Wire unit is probably (at least from what I could tell from the 2Wire.com Web site) already a router. It's possible to put another router behind it, but for small home networks, it's not something that is anticipated by SOHO equipment makers like Belkin. So, it gets a little bit tricky, and you have to know how to setup a zoned network. If you also want to port forward across routed zones from the Internet, the network addressing gets very, very specific, too.
In order to port forward, your XBox has to have a fixed IP on your LAN, and so does your router. So, for common private address ranges using Class C addresses, you might have your LAN be the 192.168.1.0 network, using netmasks of 255.255.255.0 for all machine on the LAN. Your 2Wire router's internally facing LAN address might be 192.168.1.1 (netmask 255.255.255.0) and its external address would be set to be taken via DHCP from your SBC upstream network DHCP source, which will also supply an appropriate netmask, and Internet gateway address, via DHCP as it authenticates (unless you are otherwise provisioned), which would also become its gateway address (a router getting its WAN address via DHCP should be smart enough to know that its own LAN gateway is its own WAN interface, but if it doesn't, you might have to configure this).
Your Xbox would need to be given a fixed address of 192.168.1.2 (netmask 255.255.255.0 and gateway 192.168.1.1 [which is your router]), in order for there to be a "target" address for port forwarding to send and receive packets on forwarded ports.
You would then go into the 2Wire setup, and enable port forwarding of packets on whatever ports you want (55000 and 6881?) to 192.168.1.2, which is your Xbox. All packets for these ports arriving on your router's WAN interface will go to your Xbox, and to no other machine on your network. All packets coming from your Xbox on these ports to your router, will be re-addressed and forwarded to servers out on the Internet, via your router's WAN interface, but your router will drop all packets from other machines on your LAN, on these ports, that might otherwise be headed for the Internet. As far as the rest of the world is concerned, only your Xbox is at your public IP address on ports 55000 and 6881.
Now, if you want to put your new Belkin on, in place of your SBC 2Wire, you just set this up, using your Belkin in place of your 2Wire, and disconnect the 2Wire entirely. That should be straightforward.
But if you want, for some reason, to say, add the Belkin for its WiFi capabilities, behind your existing 2Wire box, you've definitely got choices to make. You could leave your Xbox on the 2Wire, and just add a WiFi zone beside it. To do that, you'd give the new LAN zone the 192.168.2.0 network, and make the Belkin's WAN interface 192.168.1.3 (255.255.255.0 netmask, 192.168.1.1 gateway), and the Belkin's LAN interface would be 192.168.2.1 (255.255.255.0 netmask, 192.168.1.3 gateway)
To keep port forwarding going, if you plan to move your Xbox behind the Belkin, you've got to use the Belkin as a router, with port forwarding enabled, too. So, that means setting up at least a 2 zone LAN. If you leave the Xbox on the 2Wire zone, you can use the Belkin either as a router, or, if it offers bridging mode support, as a bridge, perhaps, depending on what else you want to do. In bridging mode, the Belkin would just repeat all packets on its WAN to its LAN and vice versa, without doing any address translation or routing. There's nothing to configure for a bridge.
If you did want a second routed zone, you'd make the Belkin's WAN interface 192.168.1.3 (255.255.255.0 netmask, gateway 192.168.1.1) and use, say, the 192.168.2.0 network as its LAN, so the Belkin's LAN interface would become 192.168.2.1 (255.255.255.0 netmask 192.168.1.3 gateway), and the Xbox would be moved the Belkin LAN ports, and reassigned an address of 192.168.2.2 (255.255.255.0 netmask 192.168.2.1 gateway). On the Belkin, you go into its port forwarding setup, and configure the same ports to go to the Xbox.
As I say, this should work. But, some SOHO routers don't expect private IP addressing on both their LAN and WAN interfaces, and won't allow 192.168.x.x addresses on their WAN interfaces, to prevent home users from mis-configuring them. If your Belkin was trying to "help" you in this way, it wouldn't accept this, or any, private IP address zoning scheme.
In that case, you might look into DD-WRT as a flash replacement for your Belkin router firmware, if DD-WRT supports it. DD-WRT isn't quite as user friendly, but it does a lot of functions factory firmware won't, on the routers it supports.