Then you have a great selection of CPU's that are still relevant today. X99 is likely the better choice and a 1660 V4 is the goto price/performance CPU for that platform, but there are other good models as well.
1680 V4 8core 3.4ghz
$98
Find many great new & used options and get the best deals for Intel E5-1680V4 3.40 GHz Octa-Core (SR2P8) Processor at the best online prices at eBay! Free shipping for many products!
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$100
Stepping Code: SR2P8. Cache: 20MB. Processor Number: E5-1680 v4. Processor Cores: 8. Bits (Instruction Set): 64-Bit. GRAMILL LLC d/b/a PC Server and Parts. Any problem with an item must be reported within warranty period.
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1660 V4 8core 3.2ghz
$35
Find many great new & used options and get the best deals for Intel Xeon E5-1660 v4 3.2GHz 8 Core 20MB SR2PK LGA 2011-3 CPU Processor at the best online prices at eBay! Free shipping for many products!
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Also $35
Intel Xeon E5-1660 v4 SR2PK 3.20GHz 20MB 8-Core LGA2011-3 CPU Processor.
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There is also a the 10core 2689 V4 3.1ghz.
$90
With a clock speed of 3.10 GHz and a large L3 cache of 25 MB, it can easily handle even the most demanding applications. The processor is compatible with LGA 2011/Socket R and has a TDP of 165W.
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Depending on what you want, lots of good options.
Now if you want to go with X79, the costs are lower, but the performance doesn't suffer much.
2667 V2 8core 3.3ghz
$18
Make sure you are buying the correct CPU! F3-Tested for Key Functions, Category 5 R2v3. Generic picture used. Bare CPU/Processor ONLY. Hard Drives. Item details.
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# of QPI Links 2. Bus Speed 8 GT/s. Max Turbo Frequency 4.00 GHz.
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Or a 2690 V2 10 core 3.0ghz
Socket Type: LGA 2011. Processor Type: Xeon. Processor Model/PN: E5-2690 V2. PlusBoards is a division of UNIXSurplus, a large Server dealer in Silicon Valley, California. We specialize in used servers and switches.
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All the above listed CPUs have AVX instruction sets.
Depending on what board you want, your CPU options are very good.
BTW, What motherboard where you looking at?