Large database help

Hi everyone, I'm more or less new to PostgreSQL and am trying to setup a
rather large database for a data analysis application. Data is collected
and dropped into a single table, which will become ~20GB. Analysis happens
on a Windows client (over a network) that queries the data in chunks across
parallel connections. I'm running the DB on a dual gig p3 w/ 512 memory
under Redhat 6 (.0 I think). A single index exists that gives the best case
for lookups, and the table is clustered against this index.

My problem is this: during the query process the hard drive is being tagged
excessively, while the cpu's are idling at 50% (numbers from Linux command:
top), and this is bringing down the speed pretty dramatically since the
process is waiting on the hard disk. How do I get the database to be
completely resident in memory such that selects don't cause hdd activity?
How do I pin how exactly why the hard disk is being accessed?

I am setting 'echo 402653184 >/proc/sys/kernel/shmmax', which is being
reflected in top. I also specify '-B 48000' when starting postmaster. My
test DB is only 86MB, so in theory the disk has no business being active
once the data is read into memory unless I perform a write operation.. What
am I missing?

I appreciate any help anyone could give me.

-Xavier

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