Rather than rewrite the engine behind your site, I suggest you do a little research on mod_rewrite for rewriting your urls -- without making any changes to the backend engine, you could probably make very plain looking urls. You would only need to determine a scheme to allow easy decoding. You would need to change your script slightly to output urls with the right format.
Something like:
http://www.naughtienymphz.com/nnsexl...aycat&catid=27
could be remapped as:
http://www.naughtienymphz.com/nnsexlinks/Amateur
In your .htaccess:
RewriteEngine on
RewriteRule ^Amateur /index.php?action=displaycat&catid=27 [L]
I didn't look at pagination, but, you might have to do a little creative work there.
Docs for mod_rewrite:
http://httpd.apache.org/docs/mod/mod_rewrite.html