Configuration/Tuning of server/DB

Using information found on the web, I've come up with some configuration
and tuning parameters for a server/db that we will be implementing.  I
was wondering if I could generate some feedback as to configuration and
tuning so that I could compare my estimations with those of others.

Host is AIX 5.1 with 4 cpu's and 4 GB ram.  Postgresql will be sharing
this machine with other processes.  Storage is an EMC storage array.
The DB itself is very simple.  Two tables, one with 40-45 columns (
largest column will likely contain no more than 32 chars of data ), the
other with less than 5 columns ( largest column will contain no more
than 20 chars data ).  Expected transactions will be along the order of
~600K +- 100K inserts and ~600K +-200K updates per week.

Thanks