What you really need to do to ensure compatibility with your current RAM and your motherboard is visit your motherboard's website where you should find the QVL (qualified vendors list) of RAM for your specific board. This is a list of RAM the maker has tested and certified as compatible with that specific motherboard. Because there are too many RAM makers making too many models, the motherboard makers cannot test them all, so you don't have to buy listed RAM, but MUST buy RAM with the same specs as listed RAM.
Secondly, what makes RAM combatible for use with other RAM?
Type, timings, speed, density, and quite often, strategic placement of tongue along with a little touch of "FM", where FM stands for a certain type of magic!
It is the last two in that list where gcavan's suggestion to get the same brand and model # comes into play. Technically, if the RAM "tightly" adheres to "industry standard" design specs, and the board/chipset is designed to be compatible with that RAM specification, all "should" be good, regardless the brand - in theory
But theory and real-world don't always jive, and that's the problem. The RAM can test good, work fine by itself, then fail when put to real-world use and/or when paired with other, slightly different, but supposedly compatible RAM.
So again, in theory
, you "should" be able to use one pair of one model of compatible RAM, and another pair of a different model of compatible RAM and they "should" work together. And there's a good chance they will, especially with the latest generation of motherboards and memory controllers - with are much more tolerant of mismatched RAM than just a few years ago. But there is still a chance they will not work together - so using the QVLs minimizes that risk.
Then there are the memory wizards. These are made by the RAM makers to match compatible RAM with your board. Most makers have them, with some being automated where they scan your computer for details, others require you to enter your motherboard details. The most popular is probably the Crucial
Memory Advisor. G.Skill has the G.SKILL
For future reference, when upgrading your computer or building a new computer, board makers also have QVLs for CPUs - and unlike RAM, motherboards are CPU brand specific. That is, the motherboard will support ONLY Intel or
ONLY AMD processors, and only those Intel or AMD CPUs with a specific socket type. That greatly limits the field making it easy for board makers to list all compatible CPUs for that board. Therefore, unlike RAM where you can go off-list, you must buy listed