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Memory allow a computer to store information for future usage. This artile relates to the so called internal memory, another form of memory is the external memory. Examples of external memory are a hard drive an USB stick, a CDrom player, etc. Besides internal- and external memory, the operating system also seperates fysical from virtual memory.

Memory, why it is usefull

The biggest advantages of the internal memory are it's fast acces time and it's large transfer rates. Modern memory reaches transfer speeds up to 10,000 MB per second, compared with a modern hard drive that reaches 90 MB per second , this is an obvious difference. The disadvantage of internal memory is that when the PC is shut down, all data is lost. Therefore, the internal memory is acts as a temporary buffer for the processor and not as a place to store your documents and pictures.

The internal memory discussed above is known by the operating system as Fysical memory. If there isn't enough memory available software programs will not function. In order to deal withthis, the windows operating system found a solution in the so called "virtual memory". This virtual memory is not a chip that can be seen and touched like the internal memory, it is just a file on your hard drive. The advantage of this virtual memory is that when more memory is required then the physical memory can offer, the pc will continue to operate because the data can be stored on this virtual memory. The disadvantage in this scenario speaks for itself, because rather then the previously mentioned 10,000 mb/s, the PC now has dothe same task with just 90mb/s, which is more than 100x slower! In practice you can notice this when swapping between different software (alt+tab) or changing between different tabs in your browser; when the virtual memory is used you will have to wait 1 or more seconds before it become visible. This is caused by the fact that either the pc has to generate the picture displayed on your display all over, or because it has to retrieve the data from you hard drive which is slower but also has a much larger acces time (10 ms instead of 30ns, 1000x higher).

Point of attention: Generally speaking more memory means a faster response of the computer, however too much memory does not make your PC faster. Memory acts as a bottleneck to the performance of your pc, when it is not avaiable your pc will be slower, but when there is an abundance it will not help at all. Just think of memory as the fuel of your pc, without it you can drive (by pushing the car), with it you can drive a lot faster. But more fuel does not mean your car can drive 300 miles per hour. 

Does my pc require more memory?

You can easily check how much memory your PC uses how much of it's memory is allocated to the virtual memory (which  you do not want to be used).

• windows xp:
  -press keys ctrl+alt+delete, now choose "Windows Task Management"
  -choose the performance tab
  Now you can see by Kernel Memory (KB) -> "In swap" how much virtual memory you have, and in "Physical Memory (KB) -> Available" how much memory is available. If the final value (available) is lower than 100,000, you probably need more memory. Another criteria is that the value of "Peak" in total memory usage has to be lower then the total avaiable of the fysical memory.
  
  
• windows vista
  press keys ctrl+shift+escape or ctrl+alt+delete en then choose taskmanagement, then choose tab "Performance". Now you will see the following:
  
  If the physical memory is used for more then 80% it is advised to expand your memory capacity.

Indications and guidelines for a shortage of memory

• Using the paging (see above)
• rattling hard disk (may also be caused by fragmentation, url follows)
• Slow repsonse of your browser
• long waiting times when changing applications (alt + tab)

As a guideline, for a PC equiped with Windows XP , 1GB (1024 mb) is required nowaydays. For Windows Vista at least 2 GB (2048 MB) is recommended (Windows Vista itself already uses between 600 to 900 mb of memory)

Ok, I do need more memory but what type?

Perhaps you realised that you do not have enough fysical memory avaiable. If you want to do something about this then the first step would be to identify the type of memory your pc uses. At this time it is very advantageous to buy DDR2 memory, you can order an extra 1 GB for as just 12 euros. The following types of memory are currently used in the pc industry:

• SD-Ram (old tech)
• Rambus (old tech, for first generation pentium 4)
• SO-Dimm (old tech, laptop memory)
• DDR (dual data rate memory, +/- 2001-2006)
• DDR2 (improved DDR memory, +/- 2005-2008)
• DDR3 (latest DDR variant, +/- 2007-present, expensive)

If this all looks to complicated then the first place to look woukld be the show where you are planning to purchase your memory; they can advise you. If you are planning on ordering via the Internet then there are several other ways to figure out what type of memory your PC uses:

1)motherboard manual
Search in the manual for "supported memory" or "Vendor qualified list" (VQL)
2)use software that makes a hardware report of your pc
Download 1 of the following tools, these tools investigate which hardware is in your pc, so here you can check which typ of memory is currently installed:

Sisoft Sandra: http://www.sisoftware.net/index.html?dir=&location=downandbuy&langx=en&a=
Everest: http://www.softpedia.com/get/System/System-Info/Everest-Home-Edition.shtml

(picture with example)

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Advanced knowledge:

Is there a maximum to how much memory my pc can handle?

Yes there is, there are two factors that determine the maximum usable memory:

1) the motherboard
the motherboard has a certain number of slots avaiable to place memory modules, each slot also supports a maximum amount of memory, for example, 4GB, 8GB or 16GB. Suppose you have 2 slots are already filled with 2 module of 512mb (total of 1 GB), now no addition slots are avaible to place extra memory modules. However you can buy new memory modules, however this means you will have to replace one of the old modules in the process. For example you could replace the 2 modules with 2x 1GB or 2x 2GB.
2) the operating system
Most operating systems run at 32-bits. However there are also 64bit variants released; windows xp 64bit (not well known) and windows vista x64.

A 32bit operating system only supports a maximum of 4 GB due to technical reasons. Because the various internal components of the computer memory also claim a portion of this, in reality with 4 GB modules installed +/- 3.2 GB will be avaiable. Even if you put 8 GB in your motherboard you will only see 3.2 gb in your operating system. A 64bit operating system has a much higher limit (128 GB), so for now with a 64bit operating system you can place as much memory as required.

Is it hard to install memory myself?

No it is not hard to install memory yourself, however realise that memory is extremely vulnarable to damage from electro static discharge or ESD (url). Beside an ESD guide PcInside also wrote a guide for placing memory (url)