I'm sorry for your lost baby girls, and congratulations on your boys!
I've often wondered how children feel who were born the "wrong" gender. In other words, when parents say, "We're trying for a girl" and then the baby is a boy, does he feel like he wasn't what the parents wanted, right from the start?
If you want to have another baby, by all means do so, but I think if you go into it wanting one gender or the other, that is not the right reason to have another child. My husband and I have 2 girls, and thought about having a third child (I think deep in their hearts, most men do want a son), but ultimately we decided that we weren't willing to bring another child into the world only because we hoped to have a boy - because the chances are 50/50 that it would be a girl, and no baby deserves to be the source of disappointment, even for a moment. Just my two cents. =)