I’ve moved house and being nowhere near the router that connects us to the net, I had to purchase an Xbox Wireless Adapter (rip-off!). Once it was setup, I had to test the network connection. For some reason UPnP wasn’t working even when enabled on the Linksys WRT54GR, so I had to set up port forwarding. The ports needed by Xbox Live are as follows:

• TCP 80
• UDP 88
• TCP & UDP 3074
• TCP & UDP 53

I found those details on the official knowledge base article found here. Hope that helps!