Set a default page template to load for multi-site

I dont think there is anything like page_template=> but you can just specify the template like this:

update_post_meta($id, "_wp_page_template", "new_template.php");

http://codex.wordpress.org/Function_Reference/update_post_meta
or using your code above:

$wpdb->insert( $wpdb->postmeta, array( 'post_id' => 2, 'meta_key' => '_wp_page_template', 'meta_value' => 'new_template.php' ) );