From a processing performance standpoint, none of those tasks require hardly any power whatsoever. Within Active Directory, there really isn't a concept of a Primary and a Backup domain controller anymore. They are simply just domain controllers.
There is no need for more than about 2GB of RAM for the setup that you mention. The biggest area to concern yourself with is high speed hard drives and sufficient capacity. Obviously the amount of data you will store and how many employees you have have accessing it will be the determining factors.
You might even consider making the domain controller a virtual machine. This way backups and the like are super easy and you can quickly restore or snap back if something isn't planned right at the start.