index bloat problem

I use Postgres 8.1 on linux
I have several tables to which I need insert about 200-500 records per
minute.
Records contains timestamp (actual time), and this timestamp is part of
primary key and index.
I need to keep data for 1 month.  I daily delete data older than 1 month
and than run vacuum analyze.
Size of primary keys and indexes still grows.
(After 2 months database size is about 35G and vacuum runs about 20 hours.
When I dumped and restored database, new size was 25G. Indexes and
constraints had half size)

Can you help me how to stop index growing or reindex database if I CAN'T
STOP writing records?

Thanx,

Monika

Here are my settings in postgresql.conf, that differs from defaults

shared_buffers = 2000

max_fsm_pages = 400000
(it isn't much, but there isn't logged in vacuum log that it needs more)

vacuum_cost_delay = 100
vacuum_cost_page_hit = 6
vacuum_cost_limit = 100

wal_buffers = 16
checkpoint_segments = 8

autovacuum = off