how test postgresql.conf settings?

Hi all,

I'm testing different configurations of postgresql 8.1 running on
debian. The method I was following was:
- change the postgresql.conf
- restart postgres (/etc/init.d/postgresql restart )
- execute my test queries

I have noticed that this is not enough to flush the cache, I don't
know where it is, if at the raid controller, at OS or at postgresql
itself. Reboot the machine I don't think it's a good solution because it
takes quite long. Do you have any suggestion? BTW is there any tool to
stress the DB, I mean, create a set of queries to execute and throw a
configurable set of concurrent connections to check the performance?

Thanks to all