Questions about vacuum analyze

System: Compaq ProLiant 3000, dual-336MHz Pentium II, 512MB RAM
O/S: Linux 2.2.6 (Slackware 4.0 dist)
DB: Postgresql 6.5.1
DB on 8GB 4-disk stripe-set using md.
DB with two tables, each with about 1M rows.
Current DB size about 1.5GB

I am having trouble deciding when to run vacuum analyze, how often, and
what state the system should be in when I run it. I have found that if
I run it once a night, on a system that is quiet (no other processes
connected to the DB) it will still take several (4-8) hours.

Does anyone have recommendations regarding vacuum analyze?
Specifically:

1) Should it be run on a system that is quiet or will it run acceptably
with other processes accessing the DB?

2) How often should it be run?

Thanks,
--
Steven Wheeler
UNIX Engineering
(918) 292-4119