can't handle large number of INSERT/UPDATEs

Hi,

I am dealing with an app here that uses pg to handle a few thousand
concurrent web users. It seems that under heavy load, the INSERT and
UPDATE statements to one or two specific tables keep queuing up, to the
count of 150+ (one table has about 432K rows, other has about 2.6Million
rows), resulting in 'wait's for other queries, and then everything piles
up, with the load average shooting up to 10+.

We (development) have gone through the queries/explain analyzes and made
sure the appropriate indexes exist among other efforts put in.

I would like to know if there is anything that can be changed for better
from the systems perspective. Here's what I have done and some recent
changes from the system side:

-Upgraded from 7.4.0 to 7.4.1 sometime ago

-Upgraded from RH8 to RHEL 3.0

-The settings from postgresql.conf (carried over, basically) are:

shared_buffers = 10240 (80MB)

max_connections = 400

sort_memory = 1024

effective_cache_size = 262144 (2GB)

checkpoint_segments = 15

stats_start_collector = true

stats_command_string = true

Rest everything is at default

In /etc/sysctl.conf (512MB shared mem)

kernel.shmall = 536870912

kernel.shmmax = 536870912

-This is a new Dell 6650 (quad XEON 2.2GHz, 8GB RAM, Internal HW
RAID10), RHEL 3.0 (2.4.21-20.ELsmp), PG 7.4.1

-Vaccum Full run everyday

-contrib/Reindex run everyday

-Disabled HT in BIOS

I would greatly appreciate any helpful ideas.

Thanks in advance,

Anjan