Display Category Specific Post List in Sidbar ?

Hi,

I am putting together a blog that will have three main categories and, whenever a user is browsing a post from a particular category or the entire category listing in the main loop, I want to display the posts for the viewed category as links in the sidebar. Currently, I have:

<?php } elseif (is_category() || is_single()) { ?>
<?php query_posts('cat=XY&showposts=10'); ?>
<li><h4>Recent Posts</h4></li>
<?php while (have_posts()) : the_post(); ?>
<li><a href="<?php the_permalink() ?>" rel="bookmark"
title="permalink for this post">
<?php the_title(); ?></a></li>
<?php endwhile;?>
<?php } else { } ?>

I need a way for the second line of code where it says cat=XY to allow XY to be a variable that represents whatever category they are currently viewing. (If they are viewing multiple categories, shouldn’t show them text at all.)

I had been looking at the documentation for the_category_ID and the suggested bit of code:

<?php foreach((get_the_category()) as $cat) {
echo $cat->cat_ID . ' ';
} ?>

However, I don’t know how to make it work inside of what I am currently working with.

Thanks!

PS: I know I could create a seperate if statement for each category and then manually assign the ID tags within each if but that doesn’t seem efficient.