So you have "T5820" in the subject line and "T5810" in the 3rd pgh.  Either way, be sure to download the owner's manual from the Dell site.  You'll especially need it if there isn't a RAM chart on the inside of the cover. Precision 5820 Owner's Manual  Precision 5810 Owners Manual  Avoid KVR, Kingston Value RAM.  Not that it never works, but has caused problems in Dell's.  Regular Kingston seems ok. Crucial RAM is guaranteed to work in Dell's and has a lifetime warranty.  A-Tech, the RAM I use, costs less but also has a lifetime warranty.  
I just purchased a barebones Dell Precision T5820 workstation and I now need to get the RAM. I am going for 64Gb of registered ECC @ 2666 Mhz, but I find big difference in prices online for regular ECC DDR4 RDIMM sticks vs. similarly spec'd memory that is labeled as "Dell compatible" or "Dell memory". Does the Precision T5810 workstation have special RAM needs? The specs available from  Dell's  website only say the following: "Four channel memory up to 512GB 2666MHz and up to 256GB 2933MHz DDR4 (RDIMM) ECC memory. (Up to 256GB 2666MHz DDR4 (UDIMM) Non-ECC memory) 8 DIMM slots. Note: Memory speed is dependent on specific Intel Xeon or Core X Processor installed."
Special requirements for T5820 RAM?