The dollar is one of the world's most stable currencies and is the only currency accepted in the US. If you're traveling to the states, you'll have no problem using your credit card. Banks are common and ATMs are inside most corner stores as well.
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It could be more beneficial to buy your dollars in the UK rather than when when you arrive in the US. This is because the pound-to-dollar exchange rate in the UK is can be better than some of the rates offered by providers in America.