mysql replication for certain dababases
I would avoid the replicate-do-db option due to the restriction on cross-database replication and users not fully qualifying a database name or not specifying "use dbname;" (it will execute on master but not replicate to slave).

Use this option instead (put it in my.cnf): 

replicate-wild-do-table=dbname1.%

replicate-wild-do-table=dbname2.%

Or you can specify the tables, replace % with table name, one on each line.
http://dev.mysql.com/doc/refman/5.0/en/replication-options-slave.html#option_mysqld_replicate-wild-do-table

This link should help get you started:
http://dev.mysql.com/doc/refman/5.0/en/replication-solutions-partitioning.html