A couple of very interesting posts can be found explaining HA and best practises.

An interesting post on the usage of HA having blade servers.

http://rodos.haywood.org/2008/12/blade-enclosures-and-ha.html

Duncan Epping explains the primary and secondary nodes role in this posting

http://www.yellow-bricks.com/2008/09/09/ha-primary-and-secondary-nodes/

Also on Yellow Bricks a posting on HA using blades

http://www.yellow-bricks.com/2009/02/09/blades-and-ha-cluster-design/