Thanks everybody for your help. We upgraded our postgres to the most recent version and reloaded the database, and have seen a huge performance increase. Our moodle installation is running better now than ever. September is typically our busiest month, and with over 16,000 users we haven't had a single complaint about performance.
We clustered a number of our high-use tables/indexes, and will probably recluster every few weeks.
We also implemented a number of postgres tuning changes which may or may not have helped, but they certainly didn't hurt. Here are the notable settings that everyone seems to talk about when it comes to performance, and how we have them set on our server with 8GIG RAM.
work_mem 20MEG
cpu_index_tuple_cost 0.001
cpu_operator_cost 0.0005
cpu_tuple_cost 0.003
effective_cache_size 6GB
checkpoint_segments 15
wal_buffers 4MB
maintenance_work_mem 1GB
shared_buffers 3GB
Thanks again, -- john