I wish I knew more about render farms as I love watching the CGI process but alas I had to do some research. I'm not completely sure on the set-up but from what I have read:
Render farms are a special form of batch processing clusters, with less of an emphasis on responsiveness - most of the processing jobs will take more than a minute. Low cost hardware and the quantity of available processing power is most important. Rendering is used in the visual effects, computer modelling and CGI industries and refers to the process of creating an image from what are essentially mathematical formulae. Rendering engines provide numerous different features, which in combination can produce a scene with the desired effects. link for quote
In my opinion I would say a i7-2600(k) would be perfect for this task multi-threaded and low price(by comparison to a x58)!