Also, bring your hotel name and address written in Chinese. The cab drivers will not know English. When you get to the hotel ask for a name card. It will have the name, logo, address, and phone number of the hotel on it. You can then give that to a cabbie.
The hotels will change cash, but only perfect foreign bills. I got told "this bill is broken" when it had a small crease in the corner. Either use an ATM in China or use one in HK and have Travelex do the conversion. There are also ATMs at the Hong Kong airport that dispense RMB. I trust those more than the ones in China. Beware that Travelex will convert your foreign currency to HKD and then to RMB if you bring them cash, doubling their fees. Use the ATM.