The more the merrier
However, it's all about how much the saved time is worth to you. When will you recover the investment, if ever.
I personally recommended here what I did - I bought on ebay an used workstation with decent CPU and plenty of memory. Added a newer video card and done!
I have a Dell T3500 with 6 core, 12 threads Xeon (similar to first gen i7-970) and is very fast (for example my Windows 7 CPU benchmark is 7.9 of 7.9 possible).
Dell has the T5500 that can have two of those CPU, I just didn't think I need one. EKSVid has a HP Z600, also with two of those CPU (for a total of 12 cores, 24 threads), I have heard that he has to keep it tied down or i'll run away, is that fast .
Sure now , there are variations.
A newer generation i7-49xx with 6 cores, 12 threads is faster than my 6 core. The i7-59xx that has 8 cores, 16 threads is even more furious... But might cost 10x of what I paid for mine.
And, on other hand, an 8 core AMD is slower than my 6 core intel.

It's all about money and research.
See here some benchmarks:

http://www.tomshardware.com/charts/cpu-charts-2013/-32-TotalCode-Studio-v2.5,3167.html

https://www.cpubenchmark.net/high_end_cpus.html

Thank you very much for your detailed information. Now I know what I'll buy.