The answer is "not a whole lot". But, there is a world of difference between your examples and:

<style> 
        h1  {text-align: center; font-size: 150%; 
             color: green; font-weight: bold;}
</style>
...
<h1>This is a header</h1>
[download]

Especially so if your CSS is only linked to with something like

<link rel='stylesheet' type='text/css' href='/css/site.css'>
[download]

What people are entirely failing to realize about CSS as opposed to FONT/CENTER tags is

1. The point is to separate style from content
2. Doing so makes it easier to make site-wide style changes
3. And, it allows people with visual impairments or other special circumstances to override your style choices in their client.