Code:
echo "<a href='news_content.php?id=$id&header=".str_replace(' ', '_', $hpagesheader)."' title='$hpagesheader'>"; 
Don't forget when you're getting something from the database you will need to change it back again, replacing underscores with spaces, notice the order in str_replace has been switched.
Code:
$hpageswhatever = mysql_real_escape_string(str_replace('_', ' ', $_POST['hpagesheader']));