Buy just what you need now. If you buy top-of-the-line CPU, you get 15% extra MIPS/Benchmark for 300% more money and bragging rights for 60 days.
Buy with a little thrift and buy again in 18 months at the same price bracket (3 steps down from TOL), if you need to. You will get more average power over the 3 year period for same money, as well as an off-ramp if everything is going great.
This was true with 486, Pentium, P-II, P-III, etc. CPU is rarely the bottleneck. It used to be disk i/o, then it was bus multiplier, then peripheral i/o, now it's 4GB RAM limit and how fast cores talk to each other and shared L2. A balanced high-quality system that NEVER EVER crashes is the best computer.
At least you are thinking right and optimizing for your single most-demanding application.
I like a little extra silence and at least a nod to power efficiency.