Yes, you can ... and likewise, if you had two processors, you can have a 4GB DIMM in slot A1 and an 8GB DIMM in A4, but you then must have a 4GB DIMM in B1 and an 8GB DIMM in B4 to match.
Hi, I purchased a PowerEdge T410 two years ago with the following RAM: 12GB Memory (3x4GB), 1333MHz Dual Ranked RDIMMs for 1 Processor, Optimized. The motherboard has two CPU slots, but the configuration I purchased only had one CPU slot occupied with Xeon 5620 CPU. The other CPU slot is currently empty. I need to bump up memory because I am using this server as an ESXi host for my home VoIP lab, and the number of VMs I am running are now consuming 11 GB of RAM. I need to install more VMs, so I know I need more RAM at this point.   I have not dealt much with computer hardware in the past 13 years, and so I have not kept up with all the changes and innovations. I am still very comfortable upgrading parts, but I am not so good at matching the correct parts to the correct motherboards anymore. So, I need help. From what I understand, this chipset accepts up to 32 GB of DDR3 RAM for each CPU. I believe there are four DDR3 sockets next to each CPU. Because I only have one CPU installed, I can only occupy DDR3 sockets next to the populated CPU slot. So, all further questions are about the four DDR3 sockets next to the CPU slot currently occupied with Xeon 5620.    When I purchased this server, I remember reading that the second DDR3 socket (socket 4) in channel 0 (currently empty) could not be used until BIOS was upgraded. I purchased this server in April or May of 2010, and the BIOS upgrade that would enable this DDR3 slot was not going to be available until some time in the summer of 2010. Because I did not need that memory socket at the time, I never updated the BIOS. I don’t want to replace the existing three 4GB DDR3 chips installed in memory sockets 1, 2, and 3. What I want to do at this time is to add a fourth DDR3 module in order to bump up the memory. I researched the prices, and it appears that the most cost-effective way to do this is to install an 8GB DDR3 module in memory socket 4. This would increase the memory from 12GB to 20GB, which should be sufficient for the near future. However, I want to be able to upgrade the memory further in the future by replacing the existing 4GB DDR3 modules with 8GB DDR3 modules. I have a few questions I need to ask before I purchase the 8GB DDR3 module:  1. Do I need to upgrade the BIOS in order to utilize memory bank 2 (DDR3 slot 4)?  2. The 4GB DDR3 modules currently installed in memory sockets 1, 2, and 3 are 1333 MHz DDR3 modules. Are they running at 1333 MHz or are they clocked down to 1066 MHz? 3. If I populate memory socket 4 with an 8GB DDR3 module, will the memory be clocked down to 800 MHz?  4. If the answers to questions 2 and/or 3 are “yes,” is there any reason to buy an 8GB 1333 MHz DDR3 module or should I go with a cheaper 8GB 1066 MHz DDR3 module?  5. I found the following Kingston modules that are compatible with the Dell PowerEdge T410 server: a. KTD-PE310Q8/8G: 8GB 1066MHz Quad Rank Reg ECC x8 Module: $75 (Buy.com) b. KTD-PE313/8G:  8GB 1333MHz Dual Rank Reg ECC Module: $72 (Newegg.com) c. KTD-PE313E/8G: 8GB 1333MHz Unbuffered ECC Module: $78 (Buy.com) d. KTD-PE313LV/8G: 8GB 1333MHz Reg ECC Low Voltage Module: $75 (Newegg.com) Which module should I get?  Thank you very much for your time and expertise!
Memory upgrade for PowerEdge T410