Question about Postgres

I apologize for cross-posting, but I need some help w/o too many
advices RTFM :). After Oracle and MySQL, this becomes the third
product that I need to learn to some degree, and I need a few links
which would provide a 'quick tutorial' especially for folks with
Oracle background like myself. Last time I had to deal with MySql it
took me a few days just to figure out how to login, and then how to
poke around, and then a few more to finally start writing some useful
code in whatever language they use that's similar to PL/SQL.

We are running a mail server, which for whatever stupid reason uses a
database (stupid, b/c it only uses it for web access, mail is actually
on the file system)

I'd like to know a couple of things
a) how do I access this thing as a DBA to poke around
b) how do I poke around
c) do I need to make any modifications to config file
d) what is the most common 'default' values that need to be changed
what's the best way to see what a performance bottleneck is (i
e) why this doesn't work:
[root(at)mt-adm httpd]# vacuumdb -a -f -z -q -h 10.0.1.93 -p5733
vacuumdb: could not connect to database template1: FATAL: no
pg_hba.conf entry for host "10.0.1.93", user "root", database
"template1", SSL off

Some 'details' on the server: (ps -ef)
00:00:04 /usr/bin/postmaster -i -h mt-adm.mentora.biz -p 5733 -D
/var/opt/scalix/mm/postgres/data -k /var/opt/scalix/mm/postgres/data
00:00:00 postgres: stats buffer process
00:00:00 postgres: stats collector process
00:00:00 /usr/bin/postmaster -p 5432 -D /var/lib/pgsql/data
00:00:00 postgres: stats buffer process
00:00:00 postgres: stats collector process

The above is confusing.. do I have 2 instances (or databases) running
on different ports? Should I shutdown the default one? (Scalix is the
product that uses the db)

Should I be 'playing' with
/var/opt/scalix/mm/postgres/data/postgresql.conf ?

[root(at)mt-adm httpd]# du -skh /var/opt/scalix/mm/postgres/data
276M /var/opt/scalix/mm/postgres/data

# "database" is rather small

# there are no more than 20-30 users on the server at any given time
# the disks are fast (50megs/sec, RAID10, SCSI)
# memory is big 8g
# cpu count is 2 with hyperthreading (it's a dell 2650)

[root(at)mt-adm data]# rpm -qa|grep post
postgresql-libs-7.4.17-1.RHEL4.1
postgresql-server-7.4.17-1.RHEL4.1
postgresql-7.4.17-1.RHEL4.1
scalix-postgres-11.0.4.25-1

Files that seem important:
/var/opt/scalix/mm/postgres/data

[root(at)mt-adm data]# cat pg_hba.conf|grep -v \#
host scalix scalix 10.0.1.201/32 md5
local all all ident sameuser

postgresql.conf (comments taken out)
max_connections = 100
shared_buffers = 1000
# bunch of locale params en_US.UTF-8'

http://www.scalix.com/forums/viewtopic.php?t=7809&highlight=sharedbuffers

There are some suggestions here, but they seem rather high (I did look
at some of them ,and the majorify make sense tweaking, I don't think I
fully understood the vacuum paramter)
.......
We run Oracle 9iR2,10gR1/2 on RH4/RH3 and Solaris 10 (Sparc)
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