Number of posts on category pages different from front page

I’m stuck. I’m running WordPress 1.5. I want 4 posts to show on front page (which is working), but I want 20 posts to show when someone clicks on any and every category (I have a special category.php page). I believe I need to do posts_per_page=20 somewhere on my category.php page and replace something, but not sure where and what…

I tried using Matt’s Custom Query String 2.6, but it would be inefficient for me as I would need to set it for each category (unless I missed something).

The pertinent part of my category.php page looks like:

<?php
$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
query_posts("paged=$paged&cat=$cat&orderby=title&order=ASC");
?>
<?php if (is_category()) { $posts = query_posts($query_string . '&orderby=title&order=asc'); } ?>
<?php if (have_posts()) : while (have_posts()) : the_post(); ?>

I know it’s probably something very simple, but I think I’ve really confused myself in looking for answers. Guidance?