Heat oven to 375°F. In medium bowl, combine peaches and sugar; toss to coat. Set aside.
In food processor bowl with metal blade, process bread pieces to make about 3 cups crumbs.* Add brown sugar and cinnamon; process to mix well. With machine running, slowly add margarine until well mixed.
Sprinkle 1/3 of crumb mixture evenly in bottom of ungreased 8-inch square (2-quart) glass baking dish. Layer half of peach mixture on top of crumb mixture. Repeat layers. Top with remaining crumbs.
Bake at 375°F. for 20 to 30 minutes or until peaches are tender when pierced with fork. If desired, serve warm with ice cream.