Ideal MAXDOP & CPUs - how to trace for parallel queries?
The post on Five SQL Server Settings to Change:

http://www.brentozar.com/archive/2013/09/five-sql-server-settings-to-change/

Suggests the following:


Maximum Degree of Parallelism: Limit parallelism to the number of physical cores in a single CPU socket,
Cost Threshold for Parallelism: Start with 50 then tune up or down depending on your need.


Also: A parallel plan running in serial means that although a parallel plan was created, the needed resources for running parallel are not available.  So the plan will serially runs sections of the parallel plan until finished.