Help with ROW_COUNT()

I need to find the 5th row in a selection, ordered by a value column. Since ROWNUM takes affect after the rownum is assigned it won't work. So I tried using ROW_NUMBER(). However the following query doesn't work:

SELECT pkey, value,

     ROW_NUMBER() over (ORDER BY value ASC NULLS LAST) AS valcol FROM table1
WHERE valcol = 3 Received on Mon Dec 04 2000 - 09:12:09 CST