Preheat oven to 325 F. Grease a 12-cup Bundt pan.
Using a hand held mixer on medium speed, beat sugar, margarine & eggs, scraping bowl occasionally, for 2 minutes.
In a bowl, combine flour, baking powder, baking soda, cinnamon, salt & walnuts. Fold into wet ingredients alternately with sour cream.
Spread half of the cake batter in Bundt pan, pushing it slightly up the sides of pan.
Spoon poppy/pudding mixture over cake batter.
Gently spread remaining cake batter on top.
Bake for 45-50 minutes or until tests done. Remove from oven, place a wire cooling rack over pan. Invert & remove pan.
Serve with a dollop of Greek French Vanilla yogurt if desired or dust with icing sugar.