my current setup is:
site.com/DIR/56
what I want to do is:
site.com/DIR/56/some-string-here-about-the-page
my current .htaccess setup is:
RewriteEngine on
Options +Followsymlinks
RewriteRule ([0-9]+) index.php?id=$1
I'm not an .htaccess expert, but it seems to me your existing setup will work. What it is doing is taking the uri given and finding an integer inside it -- anywhere inside it. It then ignores all the other text in the uri, by rewriting it as index?id=$1. $1 is simply the integer found, for example, 56.
You could be more explicit and try something like this:
RewriteEngine on
Options +Followsymlinks
RewriteRule DIR\/([0-9]+)(\/)? index.php?id=$1
However, I'm reasonably sure your fancy-urls will work correctly without any change.