expressions operating on arrays

Hi,
 I would like to say:

create table test1 (array1 int4[]);
insert into test1 values ('{123,234,345}');
insert into test1 values ('{456,567,678}');
now what I don't know how to do:

-- consider contents of array:
select * from test1 where array1 CONTAINS 567;

-- APPEND '789' to array in second row:
update test1 set array1=(select array1 from test1 where array1 CONTAINS 
'567' UNION select '789');

How do I go about expressing and operating on the contents of an array ?

   Thanks,
      John

-- 
-- Gospel of Jesus is the saving power of God for all who believe --              ## To some, nothing is impossible. ##
                  http://Honza.Vicherek.com/