Setting Shared Buffers , Effective Cache, Sort Mem Parameters

Hi

We are in the process of building a new machine for our production
database. Below you will see some of the harware specs for the machine.
I need some help with setting these parameters (shared buffers,
effective cache, sort mem) in the pg_conf file. Also can anyone explain
the difference between shared buffers and effective cache , how these
are allocated in the main memory (the docs are not clear on this).

Here are the Hardware details:
Operating System: Red Hat 9
Database Ver: Postgres 7.4
CPU'S : 4
RAM : 4 gig
Datafile layout : RAID 1+0
Transaction log : on different RAID1 Array
RAID Stripe Size: 8k

Thanks!
Pallav