Long answer - by the time you buy the components to even begin to try something like this and settle for really shitty quality you might as well buy a Tivo (Tivo can be modded by the way). Alternatively you could just get a capture card (ATI All-in-Wonder, Haupage, Leadtek), let your PC do the recording, and then stream the files to the Xbox.
Search here and in the A/V forum for the many threads that discuss this - all of which basically have a 9 year old insisting that it's possible while everyone else painstakingly explains it again and again.
