postgres and cache

Greetings:

Upgraded server from a Dual PII-400 to a Dual PIII-750 with Mylex
RAID-5. We didn't really see an increase of speed, or at least not what
we expected.

  In our quest for speed we put the data onto a RAM drive and the
queries completed in a third the time. I wouldn't expect to see this
type of speed gain from a RAM drive because the tables are small enough
to easily fit into cache. So my question is...Does Postgres force a
flush to disk while it's working on particular types of queries?

Thanks!

Merrill