For now at least the coin of the realm in Greece is the Euro which is analogous to a US dollar. I think of them as dollars. Remember Euro Dollars? Well this is them only they are worth more than a dollar. But they buy less than the much maligned drachmae used to because everything is more expensive now that we use euros. 20% more? 30% more? It varies.
Euro paper money is too big to conveniently count and the coins have too many denominations and are hard to read. The only good thing about a euro is that they are good in other European countries and you wont have to exchange them if you are going to one.
Credit Cards are new phenomenon in Greece. Up until two or three years ago no one had a credit card now everyone has several and are playing the consolidate the balance into one account game. The banks in Greece aren't regulated and watched as strictly by consumer protection agencies and tend to take advantage of the customers. They are regulated but its not enforced. All this means to you is that credit cards are quickly becoming more accepted in more and more places. However, most restaurants still don't accept them and a lot of stores and smaller hotels especially on the islands don't either. Greeks like to avoid a paper trail when they can.
Since Greeks use them now too, Bank Machines are quite prevalent especially in the center of towns an even in out of the way places too to a certain extent. Your exchange rate will vary with the daily rate published in the newspaper and subject to your banks fees.