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.