Mostly read performance

I have a largely table-append-only application where most transactions
are read-intensive and many are read-only. The transactions may span
many tables, and in some cases might need to pull 70 MB of data out of a
couple of the larger tables.

In 7.3, I don't seem to see any file system or other caching that helps
with repeated reads of the 70MB of data. Secondary fetches are pretty
much as slow as the first fetch. (The 70MB in this example might take
place via 2000 calls to a parameterized statement via JDBC).

Were there changes after 7.3 w.r.t. caching of data? I read this list
and see people saying that 8.0 will use the native file system cache to
good effect. Is this true? Is it supposed to work with 7.3? Is there
something I need to do to get postgresql to take advatage of large ram
systems?

Thanks for any advice.