i suppose the easiest way to go about this would be to use some sort of FireWire to RCA plug media bridge. Then you could... record your gaming using iMovie, or, let people watch you play it in iChat, or.... alright, well, you get my point. Video via FireWire is easy and flexible...
I think there are some really great Media Bridges called EyeTV ....
http://www.elgato.com/index.php?file=products_eyetvmain2&PHPSESSID=17f0aa871c65e8d640f3be619d27cdf0
I'd go for a FireWire product, and not a USB one. You'll get a much better quality image via FireWire. So basically, Xbox signal goes into EyeTV via RCA cables, then EyeTV connects to your Mac via FireWire, and... well, your screens is hooked up to your Mac, right?
There you go.