Vacuum and FSM page size

hi

We have an installation of Postgres 8.1.2 (32bit on Solaris 9) with a DB
size of about 250GB on disk. The DB is subject to fair amount of
inserts, deletes and updates per day. 

Running VACUUM VERBOSE tells me that I should allocate around 20M pages
to FSM (max_fsm_pages)! This looks like a really large amount to me. 

Has anyone gone ever that high with max_fsm_pages?

The other question is why such a large number is required in the first
place. 
Auto vacuum is enabled. Here are the settings:

autovacuum = true	
autovacuum_naptime = 900
autovacuum_vacuum_threshold = 2000
autovacuum_analyze_threshold = 1000
autovacuum_vacuum_scale_factor = 0.25
autovacuum_analyze_scale_factor = 0.18
autovacuum_vacuum_cost_delay = 150
autovacuum_vacuum_cost_limit = 120

A manual vacuum takes very long (around 4 days), so maybe the cost delay
and limit or too high.
Any suggestions anyone?

Cheers,
-- Tom.