The Xeon E5-1603 v3 RAM speed limit is 1866, as shown on the Intel site here, which is why you are seeing that instead of 2400. And this forum thread has more information on compatible CPUs. As per the thread V3 versions as mentioned there would work but not V4. And this 5810 tech sheet also has more information but no mention of a V4 CPU.

Thanks - so reading that thread, "Both E5-16XX v3 and E5-26XX v3 family works for T5810" - so would any 16XX or 26XX from this list work? https://ark.intel.com/content/www/us/en/ark/products/series/78583/intel-xeon-processor-e5-v3-family.html  I'd be limiting to the 14 core / 28 thread options, any additional considerations I should have before shopping online?

Edit: https://www.dell.com/community/Precision-Fixed-Workstations/CPU-Compatibility-for-DELL-T5810/td-p/8145070 this poster has the same motherboard model I do (0HHV7N), would that mean I can get v4 and more than 14 core?

Hello, the E5-2680 v4 should work with your motherboard.  Remember to update BIOS to the latest before swapping out the processor and you can also use the same cooler/heatsink.

This seller offers 30 days free return and is 20% cheaper.  That means you won't lose your investment in case of an anomaly or strange quirk.  

I just ordered this one from Amazon 18 core/36 thread. I have the latest bios already, and Geekbench has records of 5810s running it.

https://www.intel.com/content/www/us/en/products/sku/81061/intel-xeon-processor-e52699-v3-45m-cache-2-30-ghz/specifications.html

I know this is a tad outdated, but it took me forever to find any answers on my specific question and I kept getting redirected to this page, so I'm going to give my experience here for anybody else who may land on this...

I was able to install an E5-2699 v4 CPU into the Precision 580 (0HHV7N)

$ lscpu
Architecture:            x86_64
  CPU op-mode(s):        32-bit, 64-bit
  Address sizes:         46 bits physical, 48 bits virtual
  Byte Order:            Little Endian
CPU(s):                  44
  On-line CPU(s) list:   0-43
Vendor ID:               GenuineIntel
  Model name:            Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz
    CPU family:          6
    Model:               79
    Thread(s) per core:  2
    Core(s) per socket:  22
    Socket(s):           1
    Stepping:            1
    CPU max MHz:         3600.0000
    CPU min MHz:         1200.0000
    BogoMIPS:            4389.34

Caches (sum of all):     
  L1d:                   704 KiB (22 instances)
  L1i:                   704 KiB (22 instances)
  L2:                    5.5 MiB (22 instances)
  L3:                    55 MiB (1 instance)
NUMA:                    
  NUMA node(s):          1
  NUMA node0 CPU(s):     0-43

@XplosivOctopus​ 

OMG your a life saver been looking for days to make sure it works before I buy did you have to update bios before you put it in ?