A few queries

Hi

I have just installed 6.5, and I have a couple of questions that I cannot find
the answer to in the documentation.

Firstly, I am using temporary tables for a number of operations. Is
there a way to direct postgresql to keep these in memory for fast
access, or will it do this anyway?

Secondly, is there a way to instruct postgresql to process a query at
a lower priority, for things like daily maintainence routines that I
dont want to have slowing down the rest of the database ops by taking
all the cpu

Thirdly, how often should I vacuum a database. Is there a general rule
as to how many inserts or updates between vacuums is optimal?

Thanks

                    M Simms