I should have mentioned I was trying to keep both the:
physical space
power usage
and the price to a minimum.

The Atom 330 built into the MB in a small formfactor case takes minimal space, only 60W power and a little over 300$ AU each.
I can get 3 or 4 of these for the price of an I7 and still have plenty to spare of all three above points.

My main machine is an I7 with 9gig of ram,  I'm trying figure out what extra rendering power I can add to this.
The only thing that worries me is the max of 2gig of ram, I'm wondering if I can minimise the OS install?

Also I was wondering how smart Rendercow is, will it just throw as many threads as the hardware will support at a render or only as many as the memory will support.