how to improve perf of 131MM row table?

Sorry for the semi-newbie question...

I have a relatively sizable postgresql 9.0.2 DB with a few large tables
(keep in mind "large" is relative, I'm sure there are plenty larger out
there).

One of my queries that seems to be bogging-down performance is a join
between two tables on each of their BIGINT PK's (so they have default
unique constraint/PK indexes on them).  One table is a detail table for
the other.  The "master" has about 6mm rows.  The detail table has about
131mm rows (table size = 17GB, index size = 16GB).

I unfortunately have limited disks, so I can't actually move to multiple
spindles, but wonder if there is anything I can do (should I partition
the data, etc.) to improve performance?  Maybe some further tuning to my
.conf, but I do think that's using as much mem as I can spare right now
(happy to send it along if it would help).

DB is vacuumed nightly with stats updates enabled.  I can send the
statistics info listed in pgAdmin tab if that would help.

Any suggestions, tips, tricks, links, etc. are welcomed!

Thanks in advance,
AJ