query posts by comment_count and category id

Hey all.
I need little help. I use wordpress latest version and there is functions to query posts from category. Also there is function to get category id.

So i can make query for exact category like

<?php query_posts('cat=30&orderby=comment_count'); ?>
<?php while (have_posts()) : the_post(); ?>
<li style="list-style:none"><b><a href="<?php the_permalink() ?>" rel="bookmark"><?php the_title(); ?></a></b></li>
<?php endwhile;?>

to get category id , code is
get_query_var('cat')

but when i try to use

<?php query_posts('cat=get_query_var('cat')&orderby=comment_count'); ?>
<?php while (have_posts()) : the_post(); ?>
<li style="list-style:none"><b><a href="<?php the_permalink() ?>" rel="bookmark"><?php the_title(); ?></a></b></li>
<?php endwhile;?>

i get error.

What is right syntax to make it works?