AS others have said, Boston would have to offer Rondo, Bradley, the 2 first picks, plus probably 1-2 future firsts.
Howard is an MVP candidate, defensive player of the year Candidate, and All NBA 1st team every year (pretty much). There are superstars better than Rondo that would appeal to Orlando. How about Westbrook and Ibaka, Bynum, Wade and or Bosh, Carmelo and Chandler, or Blake Griffin.
Its best not to even think about Howard... yet. I say yet because you never know when he or a team will mess things up.