The easiest and most reliable way is to visit the websites of the major
RAM vendors. They have RAM selection tools that will direct you to the
exact RAM needed for your make and model of computer.
I see that Ted already answered your questions, but may I explore this a bit
deeper? How much RAM do you have now? How much do you want to add? Why do
you want to do this? If you are assuming that more RAM will increase your
performance, that may or may not be true. It depends on how much you have
now and on what apps you run.