Beat together the cream cheese and powdered sugar until smooth. Add the pumpkin and beat until smooth. Stir in the finely crushed graham cracker crumbs and pumpkin pie spice. The finer the graham crackers are crushed the smoother the truffles will be. I like to pulse mine in the blender until very fine.
Add the 4 ounces of melted white chocolate and stir until smooth. The mixture will be thick, but still soft. Cover and chill in the refrigerator for at least 4 hours until cold and firm. The graham crackers will absorb the moisture, thickening the mixture so it can be rolled into balls.
After the dough is chilled, roll the truffle mixture into 1 inch balls. The mixture will still be soft, but workable. Chill the truffle balls in again until firm, at least 30 minutes.
In a deep container suitable for dipping, melt the 8 ounces of white chocolate in the microwave in 10 second intervals until melted and smooth.
Dip the chilled balls into the white chocolate and place on parchment paper to harden. You may sprinkle with additional graham cracker crumbs for garnish. Store in the refrigerator.