The cable should come with the new HD. If not, you can send for it from Microsoft. Yes, it is a 1 time transfer and has some other restrictions (one that everyone hates; You may have to re-register your downloaded content with microsoft - ie, your downloaded (and paid for) games/movies/themes will not function, they'll act like you need to pay for them again). Microsoft has the steps written out pretty good for doing the transfer, and they have some information on their site to re-register your downloaded content (if needed) after the transfer.
As a warning, the transfer can take some time to complete, and you can't interrupt the transfer. If it get's interrupted, that's it, you'll lose any information that didn't get transfered over. Save games, content (which you can re-download), etc.
Set aside at least a hour to transfer all of you content over.
