Updates on large tables are extremely slow

Hi,

I'm trying to update a table that has about 600.000 records.
The update query is very simple : update mytable set pagesdesc = -
pages ;

(I use pagesdesc to avoid problems with sort that have one field in
ascending order and one in descending order. That was a problem I had
a week ago)

The query takes about half an hour to an hour to execute. I have tried
a lot of things.
This is my setup

Linux Slackware 10.1
Postgres 8.0.1
My filesystem has EXT2 filesystem so I don't have journaling.
My partition is mounted in fstab with the noatime option.

I have tried to change some settings in $PGDATA/postgresql.conf. But
that does not seem to matter a lot.
I'm not even sure that file is being used. I ran KSysGuard when
executing my query and I don't see my processor being used more than
20%
The memory increases for the cache, but not for the app itself.

My testsystem is an Asus portable, P4 with 1 Gig of RAM.
Disk is speedy. All runs fine except for the update queries.

I would appreciate some help or a document to point me to the settings
I must change.

Met vriendelijke groeten,
Bien à vous,
Kind regards,

Yves Vindevogel
Implements