Problem with transactions

["Matthijs Melissen" <melissen(at)phil(dot)uu(dot)nl>]

I want two users to execute the following queries:

1) delete from forum where id = 'A';
1) insert into forum (id, name) values ('A', 'testa');

2) delete from forum where id = 'A';
2) insert into forum (id, name) values ('A', 'testb');

id is a unique key. The numbers designate the user (1 and 2). The problem is 
that I don't know in which order the queries are executed (I only know each 
user executes its DELETE query before the INSERT query). I can't use UPDATE 
because I don't know in advance that there exist a row with id A.

How do I prevent the queries from being executed in the wrong order and thus 
causing an 'duplicate key violates unique constraint' error?