In theory that function would scale to 9 different levels, but most browsers are happy if they can master 3 different levels of boldness. I put them all in, just to show. You can see that (at least on Opera) even the keywords 'bold' and 'bolder' show no difference. Only Normal (100-500), and Bold (700-900) work, and there's the value 600 which works, too, but makes the whole font look a bit different, I think.
So all in all, just remember font-weight:normal; and font-weight:bold;