Update based on order

Hi

  can i execute an update statement based on order,   for example
  update tab1 set col1 = running_number
  where col2 = 1 order by col3 desc

  here col1 will be geting a running number that means for   each record updated col1 will have the next number

  sample output before update
  col1 col2 col3

  5     1   Z
  6     1   a
  4     1   T
  1     2   Y
  2     2   x

  sample output after update
  col1 col2 col3

  4     1   T
  5     1   Z
  6     1   a
  1     2   Y
  2     2   x