If you have a credit card without a PIN, which is common in the US, there are many places that you will not be able to use it in western Europe, although a PIN-less credit card does work most places in eastern Europe. You can set up a PIN by getting in touch with your bank.
Most rates for foreign currency at most sources in the US are terrible. Except on my earliest trips, I never bought foreign currency before I left. You can almost always get transportation from the airport to the center on plastic. Getting away from the airport usually gets you better exchange rates. For small amounts, you can also buy something at the airport, like a newspaper or a coffee at McDonalds, pay in $ and get change in local currency. I did that a few weeks ago in Prague to get the crowns for my bus fare into the the city. While the rate may not be the best, at least you escape the ripoff minimum commission that airport exchange booths will pop you with.
As far as ATM's are concerned, you also need to be aware that an increasing number of them are charging on the receiving end and often don't tell you. Some also give the bank's own exchange rate rather than interbank rate, which may not be so good. ATM's attached to banks in the UK often have no local charge, but in other countries such as Greece, ALL ATM's have a local charge. Some guidebooks will help you identify which banks to look for with lower or no local ATM charge. A few US banks also reimburse those fees. Freestanding ATM's at highway rest stops, railway stations, and sometimes airports often have the highest local usage fees for ATM's.
It is becoming more and more common for banks in the US to whack you for 3% fees on either ATM or credit card transactions overseas. Cap One is the best way around that, and is now the only card I use overseas. Additionally most US banks, but not Cap One, add an additional transaction fee for ATM use out of network, which is $0.75 for my credit union ATM card and $2.00 for my Wachovia card.