It seems to me they are about the same, however you would say "His intentions are noble", not 'dignified'. It means that their very essence is noble. Although nowadays it is used more loosely, dignified originally carried more the idea of having gained respect, having been placed in a position of respect, due to the -ed ending which means 'it was made so'. (Like pulverized means it was made into powder, glorified means that glory was given to it. It carries a passive meaning). So the intentions cannot be dignified, they are noble.
Also noble has the additional meaning of belonging to a high or rich social class.