There are indeed hundreds of ATM machines sprawled out all over Greece. In Athens they have proliferated like wild mushrooms. On the island of Mykonos, where I had a carpet store, at last count there were 6 ATM machines strategically located all over the island and that was aboput 5 years ago so you can imagine how many there are now. These days most people prefer using these ATM machines rather than bringing cash our using travelers checks since with travelers checks you get charged for buying them and then you have to pay a comission when you exchange them.
You can get cash advances from ATM machines using your VISA or Mastercard or even with a regular bank cashcard. The amount of money you will be able to obtain will depend on your credit limit and on the particular bank you do business with. Obviously it would be preferable to have cards which were issued by big name banks like "Citibank" or "Nation's Bank", but even the smaller ones work too.
As to the cost. This depends on the agreement your particular bank has with the particular Greek bank to whom the ATM belongs. For example, I have a Citibank cash card for an account I have at a Citibank branch in Brooklyn, New York. Everytime I use the card in Greece at a Citibank ATM I get charged $5.00 regardless as to the total amount of the transaction. When I have used a different bank's ATM machine I got charged $5.00 by Citibank and an additional $2.50 by the Greek bank.
I would suggest that you call your local bank and find out what their pricing policy is for transactions at ATM machines in Greece. They would surely be able to provide you with a qualified answer.
Besides the ATM machines, now "Western Union" has established itself all over Greece so you can always get money sent over in 24 hours should the need arise.
I hope this information will be of some help.
Regards from Sunny Athens,
See also Arriving in Greece from Matt Barrett's Athens Survival Guide and see the section on Changing Money.