How do I implement FIFO?

I want to be able to have multiple clients pulling single, mutually
exclusive records from a table in the same order that they were inserted
into the table. Imagine one group of people placing objects on a conveyor
belt, while at the other end another group of people are grabbing the first
item to come off the belt.

I think I know how to lay out the table:
create table XYZ
(
insert_seq_num serial,
is_processed boolean default FALSE,
some_data varchar(10)
);

Inserts are obvious. What I'm stumped by is how to write a SELECT that will
return the next available row in a way that two clients querying at the same
time can't grab the same one.

I think I can do it with temporary tables. I.e.
select * into temp ABC from XYZ
where is_processed = FALSE and
insert_seq_num = (select MIN(insert_seq_num)
from XYZ where is_processed = FALSE);

update XYZ set XYZ.is_processed = TRUE where exists
(select * from temp where ABC.insert_seq_num = XYZ.insert_seq_num);

select * from temp;
commit;

But even if that would work, temp tables aren't supported in 6.4. Is there
another way to do this without using temp tables?

Mark
---
Mark Wright
mwright(at)pro-ns(dot)net
mark_wright(at)datacard(dot)com