I called my mother's parents Nana and Papa (later on this changed to Pete; long story) and I call my father's parents Grandma and Grandpa.
My Nana has always been Nana, to all of her grandchildren. Pete was supposed to be Ompa (that side of my family is Ukrainian, not German, so I don't know why), but the first grandchild ended up saying Papa instead, so that's how it worked out. I refer to my Nana's mother as Great Nana (she was gone long before I was born, but she lived with my mom growing up, so she's kind of like another Nana to me, even though I never knew her). My mom's sister is now a Nana as well (it's so weird to refer to her as Nana when talking to Nicholas). If I ever have any children, my mom will be a Nana as well. I don't know about my dad. Papa has negative connotations for me, but he isn't a Grandpa either. Maybe he'll be an Ompa; he's already an "Unka" as all of my cousins call him "Unka Dave". That has nothing to do with heritage, and everything to do with the fact that we don't annunciate.
I was the first grandchild on my dad's side, and I've always called that set of grandparents Grandma and Grandpa. I don't know why, and I don't know who decided that. When my dad's first sister had kids, they used Grandma and Poppy. When the second sister had kids, they used Nanny and Poppy. Somehow my grandmother manages to keep this straight and signs cards with the "correct" names. My grandfather's parents were both still alive and functional when I was born, and I called them Great-Grandma and Great-Grandpa. My grandmother's mother was known as Grandma Cline (by everyone in the family, not just grandchildren).