While RAM (Random Access Memory) seems to be a simple subject, I can tell you from experience, if you want problems that are difficult to troubleshoot, buy cheap RAM or a cheap power supply and you will more than likely regret it.
There are several reasons to upgrade the RAM in your computer such as to boost performance, increased requirements of a new software application, planning to run virtual machines, etc. Whatever the reason, you’ll need to check to make sure you have an empty memory slot available before making the purchase; otherwise you’ll have to remove existing memory replacing it with larger sticks of RAM. Sites such as Crucial.com, Kingston.com, and Corsair.com have selection tools to determine what the maximum memory your computer will handle is along with whether or not the memory has to be installed in pairs (2 at a time). I’ve used memory from all of the major manufacturers in the past, but I have found the best bang for the buck to be Corsair Value Select. It works well even when mixing it with existing RAM, although I wouldn’t recommend mixing it in the same bank if your pc requires RAM to be installed in pairs. I also recommend against mixing different speeds of RAM in a computer even if it is in different banks. Some computers are more tolerable than others to things such as mixing speeds, but as a general rule, don’t do it. Many of the computers I upgrade will be given back to customers after only a short testing period and I use these precautions because the computers need to work without issue for the end user. In my personal computer I may take chances since I can make changes on the fly, but I don’t take unnecessary risk with a customer’s computer.
Determine the maximum addressable memory amount for your computer’s OS (operating system). What’s the use of adding RAM if the operating system can’t address it? That’s the boat you may end up in unless you verify the OS can address all of the memory. Here are the Memory Limits for Windows Releases.