This is the config I'm recommending for people running cisco 877's on iinet. feel free to post back with any amendments.
config working on IOS 12.4(24)T3
How to get xbox live working on your xbox behind a Cisco
Xbox Live needs TCP/3074, UDP/3074, UPD/88 forwarded to the xbox from the outside interface of the router
Xbox Connect needs TCP/8602 forwarded
Well, TPG uses PPPoE rather than PPPoA like most ISP, and there were a few issues with the MSS before my TPG was working correctly. Now it runs like a dream, no complaints at all.
I'll post the relevant sections below