Getting Affected Rows by UPDATE statement in RAW plpgsql

What you want is not currently possible in the form that you describe, but I think you can do what you want with UPDATE ... RETURNING. See UPDATE ... RETURNING in the manual.

UPDATE <target_table> 
SET Proprerty0 = Value0
WHERE <predicate>
RETURNING Property0;

It's hard to be sure, since the example you've provided is so abstract as to be somewhat meaningless.

You can also use a wCTE, which allows more complex cases:

WITH updated_rows AS (
    UPDATE <target_table> 
    SET Proprerty0 = Value0
    WHERE <predicate>
    RETURNING row_id, Property0
)
SELECT row_id, some_computed_value_from_property
FROM updated_rows;

See common table expressions (WITH queries) and depesz's article on wCTEs.

---

UPDATE based on some added detail in the question, here's a demo using UPDATE ... RETURNING:

CREATE TABLE upret_demo(
  id serial primary key,
  somecol text not null,
  last_updated timestamptz
);

INSERT INTO upret_demo (somecol, last_updated) VALUES ('blah',current_timestamp);

UPDATE upret_demo
SET
  somecol = 'newvalue',
  last_updated = current_timestamp
WHERE last_updated = '2012-12-03 19:36:15.045159+08'    -- Change to your timestamp
RETURNING 
  somecol || '_computed' AS a,
  'totally_new_computed_column' AS b;

Output when run the 1st time:

         a         |              b              
-------------------+-----------------------------
 newvalue_computed | totally_new_computed_column
(1 row)

When run again, it'll have no effect and return no rows.

If you have more complex calculations to do in the result set, you can use a wCTE so you can JOIN on the results of the update and do other complex things.

WITH upd_row AS (
  UPDATE upret_demo SET 
    somecol = 'newvalue',
    last_updated = current_timestamp
  WHERE last_updated = '2012-12-03 19:36:15.045159+08'
  RETURNING id, somecol, last_updated
)
SELECT
  'row_'||id||'_'||somecol||', updated '||last_updated AS calc1,
 repeat('x',4) AS calc2
FROM upd_row;

In other words: Use UPDATE ... RETURNING, either directly to produce the calculated rows, or in a writeable CTE for more complex cases.