I've been looking alot into this, and as far as i know, there isn't a cable. You'll need to fill out a cable application from microsoft and wait ages for it to come.
Another way is to either buy or borrow a 512 mb card, just transfer over your saves, then re-download everything else. I don't think you need to do that gamertag transfer thing, and you'll still be using the same console. I could be wrong though.
In my view this is an unbelievably good deal. You can sell your 20GB HHD for 30ish pounds, the 3 months LIVE is 15 pound, and the headset is a tenner, meaning you break even, and end up with 40 extra GB! Definitely worthwhile.