Dzug is correct in that the exchange/charges on foreign credit card spending will be no worse than changing your money beforehand � with the advantage that you don�t need to carry large amounts of cash.
I would recommend taking enough dollars for general spending, with larger items paid for by a credit card.
A couple of tips � take more than one card, just in case one is refused for some reason.
The last time I was in the US, they did not have PIN pads in use, and may well ask for some form of ID to permit payment by card.
If it is your first trip to the States, remember that restaurant waiters expect (are taxed based on receiving) a 15% tip. The easiest way to calculate this is work out what 10% of the bill is, and add half again. If you don�t leave a tip � do not return to the restaurant.