Did you know a when you first start your computer, it isn’t the operating system that first kicks the tires - it is something called as the BIOS (Basic Input/Output System). The BIOS is a small set of software that resides on a special memory chip which is located on your motherboard.
When you flick your PC on, the BIOS software starts automatically. The set of functions that the BIOS must do include a plethora of tests - like checking the hardware and system configuration - and then running a series of programs stored on the hard drive, or any other storage device. (The fact that operating systems can be started from external storage has led to a thumb drive portability revolution.)
After the successful completion of these pre-configured tasks, the operating system (Windows or Mac) is kicked off. The operating systems then then loads itself and take over the reins to perform some useful work.
Memory: It does more than merely store Information
Computers use Random Access Memory (RAM) to temporarily hold various commands and data that the CPU is currently working on. Think of it as a paper to write down some of the numbers that come up during a calculation that have to be re-used to make that calculation complete. RAM is just like that -- receiving information, holding it, and sending it back to the CPU as it continues to process data. The reason why RAM is used for this temporary storage is because of the ease of access (these RAM modules are physically located close to the CPU), and the speed at which these transfers take place (thereby allowing for faster processing times.) This would not be possible if the CPU were to use any other form of storage for this temporary use.
The motherboard has separate sockets for these RAM modules to be inserted -- closer to the CPU as mentioned before. Each of these modules is again printed circuit boards with IC chips mounted on them. Each of these modules can hold up to 2 GB memory.
As better, faster, and leaner (more efficient) processors are developed; the need for better, faster RAM modules is always expected as well. Each successive generation of CPU has demanded RAM modules of a different type leading to an evolution of its own. While building your own PC, there are chances that the RAM modules you are trying to insert aren’t compatible with the CPU you are using. To avoid these conflicts, the motherboard sockets just won’t accept incompatible RAM modules. Of course, follow the motherboard manufacturer's recommendations when purchasing your memory.
Computer performance can be increased greatly by adding more RAM. How much to add and when is totally a different story, though. The general thumb rule to fill up the entire empty RAM module slots (if any) on your motherboard. When you run out of empty slots, replace the older module that has the least amount of memory capacity.
Also some CPUs and motherboards work better with matched memory modules from a single memory manufacturer. This may allow the memory, which is to referred to as "DDR2 memory," to run in what's called "dual channel mode" and which offers a small performance increase. The forthcoming CPUs from Intel will offer a triple channel mode. The best results with these motherboards will come from using three identical, matched "DDR3" memory modules.