All I knew about Harriet Tubman was that she worked against slavery with the underground railroad. Read her wiki entry in response to this and know a bit more about her. I'd do the same with any new face on a bill, seems like a great idea. More people should know about Jackson, he was a wicked man, I don't know if his face on the $20 has done much to illuminate that fact.
Fun fact, Jackson directly defied the rulings of the Supreme Court in order to continue oppressing natives. They decided Georgia wasn't in its right to kill/relocate the Cherokee Indians, and Andrew Jackson replied, "John Marshall has made his decision; now let him enforce it!" And went ahead with what he was doing anyways, since hey, he had control of the military.
Again, I think it's an awesome idea, I just don't think people will be using cash money even 10 years from now.