Combine pumpkin and cinnamon in med. bowl. Combine cake mix, eggs, sour cream, sugar, oil and water in large mixer bowl. Beat on low speed until moistened. Beat on high speed for 2 minutes.
Stir 2-1/2 c. batter into pumpkin mixture. Alternately spoon plain and pumpkin batters into greased 10-cup bundt pan or round tube pan. Bake in preheated 375 oven for 40 to 45 min. or until wooden pick inserted near center comes out clean. Cool in pan for 20 min; remove to wire rack to cool completely. Dust with powdered sugar before serving.