Deleting row in 7.4 takes for ever

Hello,

I'm new to postgres and the work of DB's.

I'm running postres 7.4 on RHLinux 3.

I have a table that has 5.4 million rows of data in it. 4.6 million of
these rows are just administrative messages and not necessary so I'd like to
delete them. If I try and delete them, it takes 69 minutes to delete ~650K
rows. The current postgres configurations are defaults in postgres.conf
file. I've tried bumping up the shared_bufferes in the conf file, but it
does not help. Also I am unable to start postgres is I change the
shared_buffers to a values higher then 2000.

We have a project underway to upgrade to postgres 8.2, but that will take
some time, so I'd like to delete these in 7.4 if at all possible in a
reasonable amount of time so that I can minimize production impact to
customers.

Any and all suggestion will be greatly appreciated.

Regards

Bharat