how to make PostgreSQL using "all" memory and chaching the DB completely there

Hi.

I've read through the documentation and while I've seen dozens of
options which allow to set which kind of buffer/cache/shared-mem gets
how big... I do not quite understand how to reach the following.

What we have here at the institute is a quite powerful server, whit
about 100GB RAM that has a PostgreSQL running on it (with several DBs).
Currently it's 8.4 but we shall switch to 9.x eventually.
The database is (physical) size is currently at about 30 GB.

What I'd like to have, is that Postgresql uses at MAX say 50% (~50GB) of
the available memory.
And there it should completely cache the DB for _reading_. When any
writes occur, these should still be sent "immediately" to disk.
But I guess having the DB completely cached in RAM (and given the fact
that we've got much more read requests than write requests) this should
give us quite some performance boost.

Any help/hints how to reasonable configure this would be highly
appreciated :)

Cheers,
Chris.