As rabbi of a synagogue, one of my roles is to answer questions of Jewish law. While on occasion I get questions about kashrut or Shabbat or burial and mourning, the area where I tend to get the most questions is naming. The typical question is something like the following: "My grandfather's name was Irving, but we are not sure what his Hebrew name was. We want to name our baby after him. If it is a boy we want to name him Jared and if it is a girl, Jordana. Is that OK? I was taught that it has to be the same first letter to count, but in Hebrew both I and J would be written with a yud, so we figure that's OK. Right, Rabbi?"
When faced with a question such as this, I would generally answer that there is absolutely no halacha (Jewish law) whatsoever about naming. That it is all a matter of custom, not law, and the happy parents are free to name their baby anything they wish, but they should try to be sensitive to the feelings of other family members. And they usually respond in stark disbelief, because "everyone knows" what the Jewish "law" is on this subject.
This is to some extent an example of something that our Torah portion this week warns us against. In Deuteronomy 13:1 Moses says in God's name "be careful to observe only that which I enjoin upon you; neither add to it nor take away from it." We are required to observe God's commandments, but we are not supposed to invent new ones on our own. Tradition and custom are fine, but we have to be careful to distinguish between what is actually law and what is merely custom. I wish that more of us would be as scrupulous in observance of the actual mitzvot as we are in observing customs, many of which are based in superstition. Name your child whatever you wish, but the cause of Judaism would be better served by making sure that the meal for the bris or baby-naming, and later on the Bar or Bat Mitzvah, is kosher.