bold

Comment author names and or nicknames are usually in html tags called cite.
Add this to your style sheet:
cite{font-weight:bold}
But if your theme’s css already styles cite find that in style.css and change it to font-weight:bold.
If it isn’t cite…well it could be anything and that is hard to guess. Get the web developer toolbar, a firefox add on, and use the information tab to get element information. Then select comment author’s name and find the class name and html tag. Or just view the source of the comments and try to find it that way.
In the latest default theme comment author names are in a class called .fn (cite class=”fn”) but you might not be using the latest default theme.