Broadwell-E also doesn't overclock as well as Haswell-E. While your 5930K hit 4.5 ghz, it will be harder for a 6900K to do so and it also has more cores. I think that's on the high end for 6900k overclocks that I've seen.
6950x because it still can be overclocked decently as lutjens says. If it can hit 4.2 or 4.3 GHz, it won't be at too much of a real-world disadvantage compared to a 7700k at say 4.8 GHz. How much of a difference would that 0.5 GHz and the small IPC increase matter in a game? I'm guessing not...
Yeah the 6950x is about $170 cheaper ($1580 on pcpartpicker) than it was back at its launch date, making it a slightly better deal, but it's a terrible overclocker.
If you have a 5960x already, it's either get the 6950x or keep the 5960X. The 6900K is not worth it at all (it clocks similarly...
This guy is a very effective troll. He's done a good job of working up everyone here - it's clear he's not interested in a productive conversation. Just ban him.
I would probably check out servethehome forums, they do a lot of builds with server hardware and cases. They'll probably know what fits the 4U form factor.
What's your budget? You might even consider getting one of the used dual 2670 Dell or Lenovo workstations that are pretty common on eBay. No need to worry about building a system in that case, especially since overclocking is all but impossible with these chips.
I would strongly recommend...
The 2686v3 comes into stock from time to time on ebay, either as OEM or QS (pretty much as good as OEM imo). Just keep checking every few days. Its another one of those custom chips made for a particular client. You might also consider Taobao (a chinese auction site) if your chinese is good. The...
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I would recommend getting the 2683v3 or 2686v3 if your jobs benefit from more cores. If i recall correctly the 2683v3 offers 16 cores and the 2686v3 offers 18 cores. You could also get the QS versions. Check eBay.
Can confirm this. The 2679v4 can only go up to 3.3 ghz single core, you can maybe squeak out another 100mhz (so around 3.4 ghz) through BCLK overclocking - my preliminary efforts show you can go up to 104.5 mhz (a 4.5% increase from stock). Not particularly interested in overclocking though...