Need some help.. Custom sidebar displaying all posts..

Hey I have a question and hope someone can help me figure this out. I am designing a custom one off theme for a client. On the sidebar the client wants me to display 3 categories, and all of the posts per each category underneath a heading. No big deal, I used

$args = array( 'numberposts' => 10, 'category' => 6 );
  $myposts = get_posts( $args );
  foreach( $myposts as $post ) : setup_postdata($post);

to fetch the categories i needed. This is not my question though.

QUESTION:
When a person were to click on a post on the sidebar, it would open the “single” and continue to display the sidebar and that particular post. How would I add something like “current_page_item” to that particular item in the list, so as to keep that item highlighted? Any help would be seriously appreciated..
-Marc