Having experienced resourse limitations on an HP EliteDesk 705 G2 SFF I decided to add more memory to it. It has four slots for memory and came with two 4GB memory modules. I ordered two 8GB modules from Crucial, where you can search for upgrades compatible with your particular computer model. These arrived and I proceeded to install them, hoping for a 24GB system. This proved to be trickier than I had expected. It matters a lot which memory module goes in which slot and how carefully they are inserted. With some arrangements the machine would not boot, either being totally unresponsive or emitting a series of nasty beeps. Sometimes it would not recognize one of the memory modules, showing less memory than I had (I thought) installed. The most bizarre situation was when the BIOS duly recognized 24GB, and so did Windows when I booted it, but Linux (Ubuntu) only recognized 12 GB. My Linux rescue USB disk also only showed 12GB. With a lot of trial and error I got it to recognize 24GB in both operating systems, though I have some anxiety about how long that will last. Did I insert all the modules securely?
I vaguely recall reading that mixing different sizes of memory modules, e.g. 4GB and 8GB, can be problematic. The last two times I added memory to computers it was the same GB module as previously installed and I had absolutely no trouble.