The following bit of code adds a class to the first paragraph output by the_content
:
function first_paragraph($content){
return preg_replace('/<p([^>]+)?>/', '<p$1 class="intro">', $content, 1);
}
add_filter('the_content', 'first_paragraph');
Add the above to your theme’s functions.php file.
Then in your CSS add something like:
p.intro { font-weight:bold; }
I can’t claim credit for this solution (see this thread in the WP forums) but I tested it and it worked great for me in WordPress 3.3.2. You should be able to modify it as needed to target whichever paragraph your heart desires.