Further to that last post, I would also recommend the Nationwide card - I have had one for about 2 years now, and the rate you get is definitely the best around, with no foreign currency transaction fees. A lot of credit cards hide this fee in the poor exchange rate you get, some add it on as a separate charge, but it is such a rip off whatever.
In NY in Feb, I was getting a rate of $1.91 (although then someone managed to get hold of my card details, and used it twice at the Ritz Carlton days after I was back in the UK, but that's another story).
However, I do still always take cash - you can't use a card for every purchase (bridge toll $1 from Tampa airport for one thing!) - and at least you know where you are then. It's very difficult to keep track otherwise, plus half way through your holiday, you have no idea what the exchange rate actually is!