The cheapest option at the moment is the Wind F2G2. According to the table Effendi posted above, F2G2 charges 0.0017 Euro/sec (roughly 10 cents/min) for all the Greek mobiles and landlines and 4 cents per SMS to Greek *and* international destinations.
You need to buy a Wind F2G prepaid card and then dial 1342 to change your plan to F2G2. More info and details here:
WIND HELLAS
The only "problem" you may face is that as of November 2009, all newly purchased prepaid cards need to be registered, therefore you need to have a Greek address. But I suppose that since you will be there for 2 months, you can use the address you will be staying for the registration.