Credit cards are accepted just about everywhere and they are essential if you want to hire a car as the hire company will use it as a security deposit, generally they will not accept cash.
More and more places are using 'Chip and PIN' so it is a good idea to make sure you sort out your PIN numbers before you travel. Look out though for some credit card machines, particularly at self-serve petrol stations that assume you have a US zip code as you will not be able to use them if you are visiting from overseas.
You'll also need your PIN if you want to use an ATM machine to withdraw cash though you will almost certainly be charged a handling fee.
Visa, Mastercard and American Express are widely accepted but bear in mind that the exchange rate can vary day by day when the US dollar amount is converted back into your own currency.