You are right that DB clusters and memcache would work. Even simple filesystem caching would probably work. A reverse proxy cache like Varnish would probably take some of the heat off.
You can change the length of a page of posts. So, it has to be generated each time, with a query to the database.
Even simple filesystem caching would probably work.
The problem with a simple filesystem would be concurrency. You would have to serialize writes. And, once you create a lot of individual files, you introduce new problems with the metadata.
A reverse proxy cache like Varnish would probably take some of the heat off.
I think FR is already using squid as a reverse proxy, based on some of the errors that I've seen. That may have been in the past.