autovacuum on a -mostly- r/o table

Hello,

we are running a 7.3 postgres db with only a big table (avg
500.000records) and 7 indexes for a search engine.
we have 2 of this databases and we can switch from one to another.
Last week we decided to give a try to 8.1 on one of them and everything
went fine, db is faster (about 2 or 3 times in our case) and the server
load is higher - which should mean that faster response time is achieved
by taking a better use of the server.

We also activated the autovacuum feature to give it a try and that's
were our problems started.
I left the standard autovacuum configuration just to wait and see, pg
decided to start a vacuum on the table just midday when users were
launching search queries on the table and server load reached a very
high value so that in a couple of minutes the db was unusable

With pg7.3 we use to vacuum the db night time, mostly because the insert
and updates in this table is made in a batch way: a single task that
puts 100.000 records in the db in 10/20minutes, so the best time to
actually vacuum the db would be after this batch.

I have read that autovacuum cannot check to see pg load before launching
vacuum but is there any patch about it? that would sort out the problem
in a good and simple way.
Otherwise, which kind of set of parameters I should put in autovacuum
configuration? I am stuck because in our case the table gets mostly read
and if I set up things as to vacuum the table after a specific amount of
insert/updates, I cannot foresee whether this could happen during
daytime when server is under high load.
How can I configure the vacuum to run after the daily batch insert/update?

Any help appreciated
Thank you very much
Edoardo