Tables Locks Quetion or Strictlly subsequent numbers

Hello All,

I have such a question.
I want to receive from the database subsequent numbers and I can't
afford to miss one. There must not be any missing numbers.
So the sequence is not good for me because if transaction rollback the
there will be gaps.

So I make a table with one row and the row contains one int.
Every time I update the row in Serializable transaction level:

update foo set lastvalue = lastvalue+1;
select lastvalue from foo;

This is my decision of the problem. But here is my next question.
If two apllications try to take next number at the same time one of both
transactions will abort.
The one way is to catch the error and try again, but this is what I
don't want to do.
So is there a way to escape transaction error. I read about the locks
and I think they can solve my problem.

First I thick I must change my transaction Isolation Level to Read Commited.
Then If I first lock (in ROW EXCLUSIVE mode) the table, then update and
then read - will this solve my problem.
And if two functions try to do this in the same time will the second
transaction waith until it can lock the table and then without errors to
take the next number?

And my questions:
1. Should I change the transaction isolation level to Read Commited or
Serializable transaction level is good enough (I prefer to work in
Serializable transaction level)?
2. Is my algorithm correct and will it give me secure way to get
subsequent numbers without gaps?
3. Can I use SELECT FOR UPDATE instead ot locks in this case?
4. Can I change the transaction level back to Serializable after I get
the number I want, without commiting the transaction?

Thanks for the answers in advance.

Kaloyan Iliev