Sandy Bridge render nodes....
The K-series chips are clearly trivial to overclock to the 4.5GHz range, and over 4GHz should be easily achievable even in a 1U rackmount case.
Last month I built a 6-core AMD render node, which cost about $500 in parts. For Mental Ray rendering of Maya 11 scenes, it is roughly comparable to an earlier 8-core Mac or half of a current 12-core.
Note that for this particular type of task that 6-core AMD Phenom II's are very competitive with Intel and have generally been better for the price, despite AMD's huge lag in most other areas.
AMD's extra two cores are so useful for this parallel task that even Sandy Bridge would not be much if any faster for this particular job at stock speeds.
They key word there being stock, as the Intels have far more overclocking headroom than any current AMD chip. Using overclocked Sandy Bridge parts, we're potentially looking at a rackmount Maya render node that can go toe to toe with a current 2.4GHz 8-core Mac Pro, for a hardware cost no higher than the mid-$600's.