They could have only two guys on their roster, Dirk and Marion and being as much as 23 million under the cap.
They are going to amnesty Haywood, and are currently trading Rudy and Brewer, I don't think they will pick up Beaubois/Jones options.
Which means they are a threat to Boston if Howard is a FA, and Deron is their primary target.
However Boston would have more capspace than the Mavs although by little, but barring any trades we would have young assets like Rondo,Bradley,Johnson and Paul Pierce with the possibility to resign KG and Allen.
Dallas wouldn't be able to build a good roster around Howard so to go play with Dirk and Marion is not as good as playing with Boston. I think Deron is a better fit with them.
If they could entertain Deron as much as possible and convince him to leave the Nets Howard is practically a lock to come to Boston.