The Original Ralph :
fwiw, i purchased an i7-4790(non-k) thinking as a number of the posters here have indicated. Since i've regretted not going with the K version, after seeing what performance gains i've seen overclocking the rig i use for video rendering (in my sig below), but i'm too lazy to sell the non-k and pickup a K
This really depends a lot on the capabilities of the processor, and what you'll be using it for though. A 5960X comes with rather low stock clocks, with a lot of headroom for overclocking. It has a 3Ghz base clock, but only boosts up to 3.5GHz on a single core or 3.3GHz on all cores, while an overclock can bring it up around 4.5GHz, which is a massive 36% increase in clock rates over the stock all-core boost, or about a 29% increase over the single-core boost.
The 8700K, on the other hand, will boost up to a much higher 4.7GHz on a single core, down to 4.3GHz on all cores, while overclocking will typically only bring it up around 5Ghz or so, which is just a 16% increase over the stock all-core boost, or a 6% increase over the single-core boost. The 8700K has significantly less overclocking headroom than your 5960X, so a lot more of its potential performance is already available in it's stock configuration, and the same goes for the locked 8700 with it's similar stock clocks.
And back to your 4790, it can boost up to 4.0GHz on a single core, or 3.8GHz on all cores. A 4790K, on the other hand, boosts up to 4.4GHz on a single core and 4.2GHz on all cores, so that's around 10% higher clocks out of the box, even before one gets into overclocking. However, those clock rates are already relatively close to the safe overclocking limits of the processor, so again, there isn't a whole lot of room for overclocking compared to your 5960X. I believe the 4790K might typically get up around 4.6GHz or so at a safe voltage, which would only be around 10% higher than its stock multi-core boost, or 5% over it's single-core boost. So, much like the 8700K, there's much less room for overclocking compared to your 5960X. The big difference with the 4790 comes down to its 10% lower stock clocks compared to the 4790K, which doesn't carry over to this current generation, where there's only around a 2% difference in boost clocks between the locked and unlocked parts. Even if you compared your locked 4790 against an overclocked 4790k, there's only around a 15-20% difference in clocks though, which is still significantly less of an improvement than what you see when overclocking a 5960X.
Most higher-end processors won't overclock as much as what you're seeing with the 5960X, since they're generally clocked higher to begin with. Overclocking a 5960X is pretty much necessary to get the most out of it, but it's much less important for an 8700/8700K, which have a much larger portion of their potential performance available at stock settings. Moving from an 8700 to an 8700K overclocked to 5GHZ, you're looking at spending an extra $50 or so on the processor, an extra $50-$100 on the cooler (assuming you're also replacing the 8700's stock cooler with an inexpensive tower cooler), and an extra $30 or so on a Z-series motherboard. That's about an extra $130 or more, for up to 8% better performance in lightly-threaded workloads, and 16% in heavily threaded workloads, at best, assuming some other component isn't what's limiting performance in a particular application. In today's games, for example, even with an enthusiast-level graphics card, an 8700K overclocked to 5GHz will only get around 2-3% higher frame rates than an 8700 on average, which should be a completely imperceptible difference, and arguably not worth the costs associated with getting that bit of extra performance.