@Chino de Oro​ thanks. Is my system limited to the 2666MHz speed or will these work also, the 3200MHz? HMA84GR7CJR4N-XN Hynix 32GB (1 x 32GB) 2RX4 PC4-25600 (DDR4-3200) Memory

Yes, you can use faster speed memory in your 5820 but it will be downclock to 2666 MHz as that is the support rate for Skylake Xeon.

@Chino de Oro​ do you know if the ram below would work with my setup? It is Precision 5820 with Xeon W-2145 3.7GHz and would like to expand the ram to 256GB. Extra details are further below.

 https://www.amazon.co.uk/Hynix-HMA84GR7AFR4N-VK-DDR4-2666-Registered-Memory/dp/B071184B3W

210-ANJK Precision 5820 Tower XCTO Base X 1
650-AABC No Anti-Virus Software X 1
525-BBCL SupportAssist X 1
640-BBRC Dell Precision Optimizer X 1
411-XXYB Boot drive or storage volume is greater than 2TB
(select when 3TB/4TB HDD is ordered)
X 1
658-BCUV Dell Developed Recovery Environment X 1
658-BBLB Dell Applications for Windows X 1
379-BBCY No Additional Cable Requested X 1
630-AAPK No Productivity Software X 1
620-ABFD Windows 10 Pro, WKSTA OS Recovery 64bit -
USB
X 1
619-AMRH Windows 10 Pro for Workstation (4 Cores Plus)
Multi - English, Dutch, French, German, Italian
X 1
580-ADDF Dell Multimedia Keyboard - UK (QWERTY) - Black X 1
570-AAIQ Dell Optical Mouse - MS116 - Black X 1
575-BBCH No Stand included X 1
555-BBNI No Driver X 2
555-BBJO No Additional Network Card Selected (Integrated X 1
NIC included)
414-BBBV Intel NVMe PCIe SSD (Front PCIe FlexBay) X 1
490-BFHQ Nvidia Quadro RTX4000, 8GB, 3DP, VirtualLink
(XX20T)
X 1
480-AAJX Display Not Included X 1
450-AAFM UK/Irish/MY/SG/HK/Bangladesh/Pakistan/Sri
Lanka/Brunei Power Cord
X 1
430-XYMF Precision T5820/T7820/T7920 Resource DVD X 1
429-ABER No Optical X 1
429-AABU No Cyberlink Media Suite X 1
401-ABJS M.2 256GB PCIe NVMe Class 40 Solid State Drive X 1
400-AVDR Dell M.2 carrier X 1
400-ASSO 3.5" 2TB 7200rpm SATA Hard Drive X 1
400-AKZR No Hard Drive X 4
340-BYTL English, French, German, Italian, Dutch Setup and
Features Guide
X 1
340-AGKY EUP enable info mod X 1
340-CKSZ No AutoPilot X 1
328-BCRG Ship Material Tower 5820,7820 X 1
328-BBNC Shipping Material for TPM X 1
799-AANV Dell Order X 1
817-BBCE Custom BTO Configuration X 1
817-BBBC Not selected in this configuration X 3
780-BBCJ No RAID X 1
340-ADBJ Thank You for Choosing Dell X 1
340-ACCX Shipping Docs - English, French, German, Italian,
Dutch
X 1
321-BERB Precision 5820 Tower 950W PCIe FlexBay RTX
Chassis
X 1
990-10177 Desktop - Free Freight SKU £0.00 X 1 £0.00
321-BDWI Intel Integrated controller (RST-e) with 1-2 Front
FlexBay NVMe PCIe Drives
X 1
631-ABMK Intel vPro Technology Enabled X 1
387-BBMZ ENERGY STAR Qualified X 1
338-BNBW Intel Xeon W-2145 (3.7GHz, 4.5GHz Turbo, 8C, X 1
11MB Cache, HT, (140W)) DDR4-2666
389-CGMY Dell Precision Tower 5820 Tower 950W Chassis X 1
429-ABEP Slim filler panel (no opt.) X 1
412-AALF CPU Heatsink 5820 Tower X 1
370-ADVE 32GB 2x16GB DDR4 2666MHz RDIMM ECC
Memory
X 1

PS. I will remove the 2x16GB to get the 256 using 8x32GB.