exceptionally large UPDATE

I've to make large UPDATE to a DB.
The largest UPDATE involve a table that has triggers and a gin
index on a computed tsvector.
The table is 1.5M records with about 15 fields of different types.
I've roughly 2.5-3Gb of ram dedicated to postgres.

UPDATE queries are simple, few of them use join and mainly consist
of updating records from temporary tables that contains a very
similar structure to the target.

This updates are rare so I can afford to tune postgresql just for
this large update and then return to a more balanced configuration.
I can even afford to be the only user of the DB so responsiveness of
the application using the DB is not an issue. Duration of the update
is.

Anything I can tune in postgresql.conf to speed up the UPDATE?

I'm increasing maintenance_work_mem to 180MB just before recreating
the gin index. Should it be more?
The update should be monolithic and it is inside a single
transaction. Since I can afford to be the only user of the DB for a
while, is there anything I can tune to take advantage of it?
What else could I change to speed up the update?

The triggers recreate the tsvector. One of the component of the
tsvector is taken from a join table.

I'll surely drop the gin index and recreate it when everything is
over.
I'm not sure if it's a good idea to drop the triggers since I'll
have to update the tsvectr later and I suspect this will cause twice
the disk IO.

thanks

--
Ivan Sergio Borgonovo
http://www.webthatworks.it