Advice: Multi-DB, shardb or hyperdb

Hi, I need some advice.
I have searched the net for suggestions, pros, cons but not really found anything good.

We are in the midst to setup a platform. We will have several lb webservers with shared storage, memcached servers and a larger DB server.
What we would like to have is a flexibility to add more db-servers and dbs as we go along.
Say we start with 1 db-server running 256 db's and in one year we will need to add another db-server with 256 db's.
Possible with any?

Many thanks
-tsl-