Preheat oven to 350?F. Grease & lightly flour a 13"x9"x2" cake pan; set aside. In a large mixing bowl, beat the cake mix, peanut butter and brown sugar on low speed until crumbly; set aside 1/2 cup.
Add the water, eggs and oil to the remaining crumb mixture; beat on low speed until moistened. Scrape the sides of the bowl; beat mixture on high speed for 2 minutes. Stir in 1/4 cup each of chocolate chips and peanut butter chips.
Pour mixture into the prepared pan. Combine the peanuts, reserved crumb mixture, and the remaining chocolate & peanut butter chips; sprinkle over the batter.
Bake for 40 - 45 minutes, or until a toothpick inserted in the center comes out clean. Cool completely.