I would trend to agree with over estimating CPU cores and RAM, especially if you’re going to be co-locating BES and DB2 services. Also, it’s important to configure DB2 too limit the total amount if RAM it can consume.
Another suggestion is to setup regular NMON snapshots to a daily log file so that you can track CPU, memory and storage attributes of your root BES server over time.
Are you going to use a VM for the bigfix root server?
Do you mean 64GB divided among many VMs? or a single VM?
Or is the bigfix root server going to be physical?
If your bigfix root server is going to be a VM, then you can always increase the RAM later.
In my experience, Storage IOPS is the biggest bottleneck in BigFix Root Server performance, for the DB as well as FillDB. Having enough RAM and CPU definitely matters, but it tends to be more easily addressed than storage performance.