If you build render faarm couple thing to know:

• Your nodes should have same OS as your master node (32bit vs 64bit)
   Or you may have problems with cows to moo.
  -Memory ammount should be same as on your master node, not less.
  for some hardware spec check this :http://www.geekatplay.com/phpbb/viewforum.php?f=57
  Hardware specs from my farm. it's running for some time with no problems.

Problems with Atom, it's not design for number crunching, to slow. Minimum use i7 or Xenon.
Small factor big minus, when you load 90% for 24/7 overheating a big problem.

AMD use to be good, we use them at my last job for cluster supercomputer like this:
http://www.lbl.gov/CS/Archive/news081605.html 
and i still have some opteron's in production, but at this moment, intel Xenon give best preformace. Core i7, cheap, easy to overclock, very stable. AMD big heating problem, lot power consumptions.
Also we build third fastest supercomputer in world from desktop hardware. (linux networx)

With out going brook, you can put high preformace nodes around $400