been trying to change the width with WP 3.2.1
It’s a bit tricky, i think, but if you need width up to 800px, you can change it with functions.php of the theme. Just add this code somethere:
if ( !isset( $content_width ) )
$content_width = 750;
but if you need width to be more or equal then 800, you have to change /wp-admin/includes/post.php . Search there for ‘800’ (you should find 2 instances), and change this value to something bigger, like 1000 or more and then add the strings above to Functions.php of your theme.