increment int value in subset of rows?

[postgresql 7.4, SuSE x86 linux]
I have a table "rtest" with primary key (run,seq) and other data. For a given value
of "run", seq is a sequential run of integers, 1,2,3,4.. Now I want to
insert a row into this "sequence", say run='foo', seq=2, adjusting the seq up for
all subsequent foo rows. My first thought
was just:
update rtest set seq=seq+1 where run='foo' and seq>1;
which gets:
ERROR: Cannot insert a duplicate key into unique index rtest_pkey
no surprise :-(.

This doesn't work, since the *order* of execution of these updates
is not guaranteed, and I actually would need to start with the highest
value of seq and work down. There may be a thousand or so rows for 'foo'
run, so an external loop of queries would be very expensive.
How can I increment all the seq values for foo columns where seq > something?

create table rtest(run text,seq int,data int,primary key (run,seq));
insert into rtest values('foo',1,11);
insert into rtest values('foo',2,22);
insert into rtest values('foo',3,33);
insert into rtest values('foo',4,44);
insert into rtest values('bar',1,99);

I want to shift all foo rows and insert a new one so that:
select * from rtest where run='foo' order by seq;
would get:

run | seq | data
-----+-----+------
foo | 1 | 11
foo | 2 | 999
foo | 3 | 22
foo | 4 | 33
foo | 5 | 44

--
I cannot think why the whole bed of the ocean is
not one solid mass of oysters, so prolific they seem. Ah,
I am wandering! Strange how the brain controls the brain!
-- Sherlock Holmes in "The Dying Detective"