PostgreSQL: UPDATE if the row exists, INSERT if it doesn't exists (UPSERT)

gistfile1.sql

CREATE OR REPLACE FUNCTION upsert_tableName(arg1 type, arg2 type) RETURNS VOID AS $$ 
DECLARE 
BEGIN 
    UPDATE tableName SET col1 = value WHERE colX = arg1 and colY = arg2; 
    IF NOT FOUND THEN 
    INSERT INTO tableName values (value, arg1, arg2); 
    END IF; 
END; 
$$ LANGUAGE 'plpgsql';