confused of buffers and memory settings

Hi,

we're quite new to Postgres and we're going to start using Postgres8.3
on Opensuse10.3 64 bit. We have a dedicated dbserver with 8GB RAM, and
now I'm not sure how to deal with the memory/buffer settings. The db
will be accessed heavily (~20 requests/sec.) with a read/write ration of
50:50, yes, a lot of write activity.

I thought of setting "shared_buffers" to 750000 (~6GB) but how depends
this on the kernel buffer setting in /etc/sysctl.conf (what is the
interaction between these two settings?).
I know the variable "shmmax" can be defined, but currently there's no
such entry.

The meaning of "work_mem" / "maintenance_work_mem" and "wal_buffers" is
also not clear. The maintenance_work_mem influences the size of the WAL
logs..?!?

What else are "top performance related" options for the usage scenario I
described earlier ?

any help appreciated....GERD...