BoundingBox v XNA 4.0

        /// <summary>
        /// Precalculate model's bounding box (by its vertices + bone transforms)
        /// - inspired by http://dzindzinovic.blogspot.com/2010/05/xna-get-model-vertices-for-boundingbox.html
        /// - BoundingBox can be also loaded by custom Content Pipeline processor (ModelBoundingBoxProcessor)
        /// </summary>
        public static BoundingBox GetBoundingBox(Model model, Matrix[] boneTransforms)
        {
            Vector3 min = new Vector3(float.MaxValue, float.MaxValue, float.MaxValue);
            Vector3 max = new Vector3(float.MinValue, float.MinValue, float.MinValue);
            model.CopyAbsoluteBoneTransformsTo(boneTransforms);
            List<Vector3> vertices = new List<Vector3>();

            foreach (ModelMesh mesh in model.Meshes)
            {
                // Get the transform of the current mesh
                Matrix transform = boneTransforms[mesh.ParentBone.Index];

                foreach (ModelMeshPart part in mesh.MeshParts)
                {
                    // Get the current mesh info
                    int stride = part.VertexBuffer.VertexDeclaration.VertexStride;
                    int numVertices = part.NumVertices;
                    byte[] verticesData = new byte[stride * numVertices];

                    part.VertexBuffer.GetData<byte>(verticesData);

                    for (int i = 0; i < verticesData.Length; i += stride)
                    {
                        float x = BitConverter.ToSingle(verticesData, i);
                        float y = BitConverter.ToSingle(verticesData, i + sizeof(float));
                        float z = BitConverter.ToSingle(verticesData, i + 2 * sizeof(float));

                        // Apply transform to the current point
                        Vector3 vector = new Vector3(x, y, z);
                        vector = Vector3.Transform(vector, transform);
                        vertices.Add(vector);

                        if (vector.X < min.X) min.X = vector.X;
                        if (vector.Y < min.Y) min.Y = vector.Y;
                        if (vector.Z < min.Z) min.Z = vector.Z;
                        if (vector.X > max.X) max.X = vector.X;
                        if (vector.Y > max.Y) max.Y = vector.Y;
                        if (vector.Z > max.Z) max.Z = vector.Z;
                    }
                }
            }
            return new BoundingBox(min, max);
        }