The RAM memory (Random Access Memory) is the fast memory that stores the data needed by the programs currently running on your computer. RAM memory is 30 to 70 time faster than the hard drive. Knowing the type and speed of your memory as well as the quantity of memory you have is necessary to upgrade the memory. Depending on the age of your computer, you can have SDRAM (Single Data RAM), DDRAM (Double Data RAM), DDR2 or DDR3 (respectively Double Data RAM 2 and 3).
Go to cpuid.com/cpuz.php and download the last version of CPU-Z, as Windows does not have any built-in utility that gives specific information on the RAM. Once downloaded, double click on it to install the software. Run that software. Go to the "SPD" tab.
On the top of the "Memory Slot Information," read the type of Memory (SDR DDR, DDR2 or DDR3). In the fields bellow you will find the quantity of RAM in megabytes (million bytes of information) and the speed in MHz.
Use the drop-down menu on the upper left corner with "slot #1" or "slot #2," for example, as computers can have one to eight slots where you are able to connect memory.
Open "System Profiler" by going to the "Apple Menu," selecting "About This Mac" and clicking on "More Info."
In the new window, go to the "Memory Overview" pane on the left of your screen.
In the upper frame you can read the amount of RAM per stick that your MAC has.
When adding RAM to your computer, use the same type and speed of RAM already present in your computer. If you add a memory stick that is faster than the one already in your computer, the system will run the sticks at the speed of the slowest one. You do not need to buy the same brand for all of your memory sticks.
Be sure to read the motherboard manual to know it you have any limitations on how big the memory stick can be. Some motherboards only accept stick up to 2Gb while others accept 4Gb per stick. Always ground yourself by touching a metallic surface before changing the RAM, as they are sensitive to electrostatic discharge.