Go for the newest/best Intel CPU you can afford. The difference in rendertimes between old-gen i7 and sandybridge i7 is 2×, between old-gen i3 and sandybridge i7 is 4×. I'll hopefully release a standalone benchmark scene with the next Alpha, so we can make a chart of different CPU performances.
(I also hope somebody from Intel is reading this and sends me a free Xeon Phi for development for promoting the company :D)