We’ve put alot of work into optimizing our software for low end processors. If you are using just a Bluecherry hardware compression card you could get away with using an Atom or Celeron CPU with 2GB of RAM (assuming a headless installation without the client running).
However handling motion detection on IP cameras uses more CPU and memory. If you plan to deploy 8 1.3 megpaixel cameras we would recommend atleast a i3 CPU with 4GB of RAM. Typically you will not need server based CPUs (Xeons for example) unless you have 32 or more cameras.