From what happened to a friend of mine a few weeks ago, you can always transfer our online accounts (and online content) to any new xbox, as long as you remember your passwords n such. His was stolen, used, but a couple of days into it was recovered and installed on his brothers xbox system. He actually recovered his original xbox, and put everything back on to it.
Also, you can plug in your old hard drive to the new xbox, copy the saved files to the system, then copy on to the new hard drive.