How to change money in Ho Chi Minh city?

I am going on my first trip to Ho Chi Minh City. Are there lots of money changers around for the Australian Dollar or is it easier to withdraw from an ATM? Has anyone stayed at the Tan Hai Long Hotel? I have 7 days holiday what would be the best things to see? Is it possible to go across to Cambodia for the day to see the Ankor Wat or is it too far?