First, equating clock frequency with performance makes me cry (I'm an Electronic Engineer with an interest in computer architecture).
Second, you should consider value per buck and not just bang per buck.
For example, suppose that I could get 5% better capacity/dollar with a 400GB HD than with a 200GB HD. Since 200 GB is enough for me, buying the 400GB would basically mean paying 90% without enough benefit. Buying the 400GB is more bang per buck but less value per buck.
And the justification that I should buy the 400GB because "I will need it in the future" is just a shallow excuse for consumerism. When/if I need the additional 200GB, it will be cheaper, faster, and more reliable (due to being less used) than now.
I know GHz != FLOPS, and that Hz/FLOPS in CISC processors can vary a lot, but I find that lot of the time, hertz are still a decent measurement of power.
Also, with respect to bang for your buck, I'm talking within reasonable bounds(a few hundred MHz difference)
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