ATM card will let you take out cash here and abroad at ATM machines (there are two systems in Hong Kong and you can take cash out/pay bills at any ATM in your bank's system free of charge. It will also have the EPS function (like a debit card), which will allow you to pay in thousands of places without cash/credit card. You can also use it for 'cash back' at many places. If you think you will never need cash (ha!) or prefer to spend your lunch hour queuing in the bank, then you don't need an ATM card.
Credit cards work the same here. It can be difficult to get one immediately though.
For online banking, well with my bank I just registered and had immediate access. Banks will also have smartphone apps for you to do you online banking wherever you are.