Netbackup 6.0 puts a greater processing load on the master server than previous versions that some Symantec techs recommend using a four processor box as a starting configuration for the Master server.
NOM contains a database and adds processing load to the server. To make sure NOM doesn't interfere with the backups, it is best to install NOM on a different server.
Make sure you add a server entry in your bp.conf file for the NOM server, or else the NOM server will not be able to see the Master server.
NOM seems to have greatly improved with Netbackup 6.5.