Lots of studies have shown that the best time to introduce a second language is in toddlerhood, and that kids this age have no confusion over which language to speak to whom. A friend's daughter who is working on a PhD in neurological development told me that the newest work suggests that different language centers are used for learning language before the age of 5 than after, and that language use in two or more languages is more "natural" to those who learn as babies than even in those who learn as young children.
One thing to consider is how academic kindergarten is in Cypress (I have no clue). If kids are expected to start learning to read and do simple math in kindergarten like they are here, your daughter will be at a distinct disadvantage when she starts school. I worked with a child like this when I was helping out in my daughter's kindergarten class many, many years ago. He spoke no English when he started and passable English by the end of the year, but was way behind the class in every academic subject and had trouble socializing for a while. My sister teaches special ed and works with English as a second language students and has said that as she follows these kids through school, some never quite catch up until middle or high school, and some never catch up at all.
Considering all of the above, if it were my child, I'd emerse her in Greek now. Put her in a Greek pre-school and have your husband speak and read to her in Greek at home. By the time she starts school, she'll be bilingual and on par with her classmates.