In general, the more cores the better.

Here’s some general guidelines for choosing hardware:

• Arnold scales linearly with increasing processor speed
• Arnold scales well to multiple cores
• For a rough estimate of  expected performance, you can multiply the clock speed by the number of physical cores.
• Get enough RAM to hold your typical data sets (you probably should run
  some tests with your most challenging scenes)
• Hyperthreading on Intel processors usually gives about 1.2x speedup on average.
• If possible, running some tests is the best way to choose the best hardware for you