Hey,
I assume you simply changed:
$query_string .= "&showposts=3";
to
$query_string .= "&showposts=6";
?
If so, you can add conditional code which uses the counter function (already present in index.php) and add this code:
<?php if ($counter == '4') echo '<div class="clearboth"></div>'; ?>
immediately under:
<div class="info_box box<?php echo $counter; $counter++; ?>">
<div class="box_image"><img src='<?php echo $preview_pic_mainpage; $preview_pic_mainpage =''; ?>' alt="" /></div>
<span class="meta"><?php echo $punchline; $punchline='';?></span>
<h2><?php the_title(); ?></h2>
<?php the_content('Read more'); ?>
<?php edit_post_link('Edit','', '' ); ?>
</div>
Note: I chose counter = 4 because the count is increased by 1 in the existing code just after the $counter class is added, if I had chosen 3 it would appear after the 2nd (middle) column.
Let me know how you get on :)
James