Note that the ROWNUM may not be in sequence if you do an ORDER BY. This is because the ROWNUM is assigned before the result-set is sorted. A way around this is:
SELECT ROWNUM, Col_1, Col_2, Col_3, Col_n
FROM (SELECT Col_1, Col_2, Col_3, Col_n FROM Table_X ORDER BY Col_2);
This should work in ORACLE Version 9 and 8.1.7, but I'm not sure about earlier versions.