The following was copied (and lightly edited) from this answer to this Stack Overflow question:
Recommendations from SQL Server 2008 Books Online: We recommend associating tables with the same update characteristics (such as small number of changes versus large number of changes, or tables that change frequently during a particular time of day) together under the same full-text catalog.
By setting up full-text catalog population schedules, full-text indexes stay synchronous with the tables without adversely affecting the resource usage of the database server during periods of high database activity. When you assign a table to a full-text catalog, consider the following guidelines:
Always select the smallest unique index available for your full-text unique key. (A 4-byte, integer-based index is optimal.) This reduces the resources required by Microsoft Search service in the file system significantly. If the primary key is large (over 100 bytes), consider choosing another unique index in the table (or creating another unique index) as the full-text unique key. Otherwise, if the full-text unique key size exceeds the maximum size allowed (900 bytes), full-text population will not be able to proceed.
If you are indexing a table that has millions of rows, assign the table to its own full-text catalog.
Consider the amount of changes occurring in the tables being full-text indexed, as well as the total number of rows. If the total number of rows being changed, together with the numbers of rows in the table present during the last full-text population, represents millions of rows, assign the table to its own full-text catalog.