I could spend a day writing a reply to this one but I only have 2 minutes, so:
I build my render farm out of i7 Ivy Bridge processors. They are 1U rack mountable, have 32 GB of RAM and I put in no less than a 120 GB SSD. The build is VERY successful and gives me the best "bang for buck", which is something I research at least every year. Each of my machines is about half the speed of a XEON dual CPU build but less than one third the price and easier to upgrade and work on with future needs. They also run cool, quite and headache free.
I am an authority on this subject and have built more render machines than I can remember so would love to write more but have to run. Maybe if you have some more specific questions I can answer them on my return.