Disk caching

Hi to all.

I am trying to see how PostgreSQL performance changes on the basis of
work_mem. So, I am going to execute the 22 queries of TPCH
(http://www.tpc.org/tpch/) again and again, each time for a different
value of work_mem.
Since I am interested just in work_mem variations, I should prevent each
query to take advantages from revious executions of the 22 queries them
selves. For example, taking cache advantages. So, taking into account
that the 22 queries are those http://pastebin.com/7Dg50YRZ and are
executed on tables of hundreds of MB and

1) Is it sufficient to run change the values of work_mem through psql
and running the queries again without restarting postgres?

2) Or, should I restart postgres?

3) Or, shoud I restart the machine each time I execute the 22 queries?

Thanks for your time.
Regards.

Manolo.