u mean how hard it is to do this...
there is no easy way, as the imac (and probably all mac products) does not have a video input port. only out.
however there is something called Elegato iTv, which connects via usb, using a video converter box thingy, which is connected to your source. It includes its own software, which will let you watch, record and play back the video.
Its fast enough to watch whatever you desire, and maybe fast enough to watch and record your xbox feed. (depending on your machine)
so again xbox ---> elegato itv ----> imac (via usb) ---> Elegato software --> save as movie!
it'll bring you back from around $100 - ~$300 (CDN).