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).

If you are planning on using 24x7 constant recordings, a very low end x86 based processor will be fine.  

However handling motion detection on IP cameras uses more CPU and memory.  If you plan to deploy 8 x 1.3 Megapixel 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 64 or more cameras.