You cant. The equivalent of page caching in memcached is action caching, because the request must be served through Rails. Page caching is meant to bypass Rails, so the data must be stored in a file that can be served from the server, like Nginx or Apache. The reason page caching is so fast is that it does bypass Rails entirely. Here is what the Rails documentation says:
Page caching is a Rails mechanism
which allows the request for a
generated page to be fulfilled by the
webserver (i.e. apache or nginx),
without ever having to go through the
Rails stack at all. Obviously, this is
super-fast. Unfortunately, it can’t be
applied to every situation (such as
pages that need authentication) and
since the webserver is literally just
serving a file from the filesystem,
cache expiration is an issue that
needs to be dealt with.
You can find more information here.