I read the whole post, including the edited part, I just think you shouldn't call somebody a liar, just because of his ignorance.
Your edited part, concerning the adapter to connect the Xbox to the Powerbook, wasn't completely right either. It's not a simple adapter that's needed, but video capture hardware, which is more than just an adapter (because the Powerbook only has Video out, no Video in). And it is possible to have the sound, even without extra hardware, because the Powerbook has an Audio in port.
Notice that I didn't call you a liar, even if you were partly wrong, Dude ;-)
To come back to the topic:
without additional hardware, it is not possible to capture the video signal of an Xbox (or any other Video source) with any of the recent Macs (some older ones had build-in Video in ports). For this, video capture hardware is needed, which is nonsense if it is only for using it as display.