Database topology

One issue that will develop is the usage of mySQL users and databases. My current thinking is as follows:

databases:
master site db-- contains user info, preferences, master for starting templates for new sites, site list, and such
board db--contains this discussion board
session db--if implemented, will be used in keeping track of users online, when last online, etc. Basicly non-critical info related to users and/or viewers
site db--one for each site. contains template/style info, user style preferences, news content, articles, screenshot db,etc.

There will be a master account with access to all related databases which administration code will use to set up sites and gather data.
There will be an account to handle user management, and such with access to the board and session dbs, master site db, and probably the site dbs.
There will be a board account with board db and session db access, and read-only access to parts of the master db.
There will be an account for each site with access to its own site db, session db, and read-only access to parts of the master db.

The intention of this setup is to minimize risk of one part of the network messing up another part.

This is in no way set in stone, and I would like advice.