Rendering is one of the few aspects that uses to advantage as many cores as you can get. Intel Core i7-3960X Extreme Edition 3.30 GHz 6-Core Hyperthreading Processor would be the best possible but is ludricrously expensive and poor value for money. An i7-3770 at about £260 has 4 cores and 8 threads and is reasonable value if you have the money. An AMD FX-8150 8-Core CPU offers reasoble value for money.