More numbers -
Since no one knows what will happen in 3 years, let's look at how people who made the same decision 3 years ago are doing, since we DO know what's happening now.
This is Newegg archive, from mid-Jan, 2011 (that's when Newegg started selling Sandy Bridge) -
https://web.archive.org/web/2011011...ategory.aspx?Category=34&name=CPUs-Processors
The equivalent choices back then would be i5-2500k and i7-2600k.
2500k is 3.3 GHz, 6MB cache, no HT, $224
2600k is 3.4 GHz, 8MB cache, HT, $329
The difference is $105.
The performance differences aren't really that great looking back now (3 years after you bought the chips), so if you are upgrading the 2500k now, let's say you'd get 1 more year out of the 2600k.
Now if you look at the classified section, the 2500k goes for about $150, and the 2600k will probably go for less than that after another year... maybe $130?
Both CPUs are still definitely adequate, but if you insist on upgrading, if you went for the 2500k back then, you have $255 now to spend on a new CPU. That's a 4670k. A 4670k is faster than the 2600k, is much more power efficient, and has all the cool new instructions.
You get $130 to spend on a new CPU once the 2600k reaches the same point in a year. I don't even want to look up what kind of CPU you can buy with $130.
Always buy what you need NOW, or at most in a few months. Not in a few years.