Hong Kong is a major financial center so there are no restrictions in terms of how much currency you can bring in or take with you when you leave, The Hong Kong Dollar (HKD) has held its own in the world currency market so the exchange rate hovers around 7.8 HKD to 1 USD.
There are money changers in all the tourist areas and they're open everyday, but their exchange rate usually includes a 5% commission. All the major credit cards are accepted, but it's always a good idea to write HK in front of the dollar sign if it's not printed on the receipt. Some shops like to charge a surcharge to offset credit card fees, so be aware of the total before you sign any receipt. There is no sales tax in Hong Kong, but there is a 3% government tax on hotel rooms. Tipping is not mandatory, but restaurants and hotels usually add 10% to the total bill as a service charge. Tips are accepted, but most service people don't expect one.