Which credit card should a traveler take to Italy? #1: Visa
Contrary to the old commercials, you can leave home without American Express (www.americanexpress.com) and never notice the difference.
Visa (www.visa.com) and MasterCard (www.mastercard.com) are much more widely accepted in Europe these days, partly because they've partnered with major European cards, and partly because AMEX charges higher fees to the merchants and is slower in paying them, so many small family businesses have stopped accepting it, arguing—with a good point—that nowadays "Everybody has Visa!"
Diner's Club (www.dinersclub.com) is accepted in many places (though usually, as expected, only the pricier joints). So is Japan's JCB (which I throw in there in case any Japanese people happen to be reading this.).
Discover or any other card will only elicit a raised eyebrow of curiosity and shake of the head from your waiter or merchant.
Leave any other card at home.