Overclocked i5s (2500k/3570k) are still viable in many games. The thing is, they are running at their limit in others, especially in many open world games. Spring of 2016 my old mobo died and I had to buy new stuff, at this point I was a bit annoyed because I was under the impression that my good old 3570k (4.3ghz) would be able to bring me through the next 2-3 years.
To make a long story short: I was shocked about the performance gain.
Just Cause 3, GTX 980, 3570k (4.3 ghz), 53fps
Just Cause 3, GTX 980, 6700k (4.3 ghz), 94 fps
Just Cause 3, GTX 980, 6600k (6700 without HTT, 4.3 ghz), ~ 88fps (sorry don't have the pic anymore).
That's of course an extreme example but games like Witcher 3 and GTA V also ran better (~30fps). That's huge in my opinion and I'm very much under the impression that it's about time to say goodbye to our old i5s.
Old i7s (2600k/3770k) seem to hold up a lot better. For example a 3770k can outperform a modern 6600k at stock clocks.
THAT said, I'd still go for a new build instead of buying a used 3770k. Just because they are still relatively expansive, old and you never know how reckless the previous owner was. Also: If your mb dies you'll be in trouble. It's getting harder and harder to find good mb with oc features for old cpus.
But kabylake (more or less higher clocked skylake, with better iGPUs) and (Ry)Zen are around the corner. Waiting for them before making a decision could be worthwhile. First Ryzen stuff looks promising but there are no game benchmarks yet besides a pointless 'look it can run BF1 at 4k/60 when paired with a Titan X (Pascal)'.
