This is the code for giving a new template for post for a specific category. this should go to functions.php
add_filter('single_template', create_function('$t', 'foreach( (array) get_the_category() as $cat ) { if ( file_exists(TEMPLATEPATH . "/single-{$cat->term_id}.php") ) return TEMPLATEPATH . "/single-{$cat->term_id}.php"; } return $t;' ));