table locking

Hi

I'm having trouble getting my head round postgres's locking. I've read
through all the docs but cant seem to find the right thingthat does what I
want...

Basically, there are a few tables in my system where a SELECT is done prior
to an INSERT to avoid duplicating certain data and so forth.

So, I want to lock the table for the transaction that does the stuff above so
no more than one can run at a time (essentially making it SERIALIZABLE).
But, I dont want to block other readers from SELECTing from the table at the
same time...

This is the classic multiple readers, single writer problem yes? I'm sure
its doable, but I cant work out which locking / transaction isolation levels
to use.

Thanks

--
Shane