Regarding credit cards: be aware that Discover/Novus cards are not accepted in Spain. Be sure to bring Visa and/or MasterCard.
As for cash, use your ATM card as much as possible. It'll usually get you a better dollars/euros exchange rate than the money changing businesses offer. But remember that you'll be charged a service fee each time you take out money, so be sure to withdraw a couple of days' worth at a time.
Buen viaje!