memcache session sessions

Hi,

Please tell me which module I can use as cache system for authenticated users but not Authcache (it doesn't work cause the problems with phpbbforum module)?
For anonymous user I use Boost, but it seems not be recommended to enable the normal Drupal caching with Boost, is this right?

At the moment I installed:
Pressflow
Cache Router (via Memcache)
Path alias cache
Locale Cache
Authcache - not working

Another thing, it is good idea to replace Cache Router with Memcache API http://drupal.org/project/memcache?

http://drupal.org/node/1131468
For sessions caching it says:

Hello everyone!
I'm rather new to this group and I need some advice.
I'm trying to build a high performance multisite in EC2 cloud and I have the following question: How can I use memcache for storing sessions and still mantain the "Who's online" functionality and session persistance.

There's a good article that talks about this (sorry I've lost the link) that propose a dump of memcache sessions to DB every N seconds, but I can't find an implementation and I'm thinking in implementing a kind of cron job to do it.