A question about efficiency

I am developing a site which will be very dynamic. Most of the displayed
content will come from a database (MySql) via PHP.

Some types of pages will change frequently (something like a forum page
which is always having new content added to it) and others will only change
once a day (a new news item gets added, etc.).

Although I can optimise my queries and my PHP scripts, there is still the
potential for the site to get congested, once a large number of people start
accessing it.

The question is how do I make sure it performs well? There are several
options I might consider, such as:
- don't do anything, but rely on server caching. Is PHP / Apache caching
any good?
- Periodically "publish" my PHP-generated pages to flat HTML pages and serve
them up instead of the PHP.
- Other things?

Any thoughts?

--
Stephen Oakes