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RAM Memory
RAM (Random Access Memory) is critical to the performance of your PC or Laptop, its the memory the computer uses to load . run your applications. Not enough and your computer will be frustratingly slow and prone to the occassional crash, particularly if you load more than one application at a time e.g. An internet browser and an Office package.
For any desktop PC I would opt for a minimum of 1GB (GigaByte), this should be more than enough for basic applications, the internet, digital media and playing games. If your looking for more performance then 4GB should do the trick!
For a Laptop, 1GB is again the minimum but try and get 2GB if you can.
RAM comes in different speeds, however most new computers are using DDR Ram (Double Density Rate). Its always worth checking how many RAM expansion slots your potential computer / laptop has. Applications are becoming more RAM intensive and its likely that you computer may need 4GB to work well with tomorrow's applications.
Memory Upgrade
If you are looking to upgrade the memory in your PC and am not sure whether you have 168 or 184, DDR 1 or II, what your voltage is etc. Then I would recommend using Crucial (Link on the top right), they have a small application which scans your PC, identifies the memory that you currently have and then recommends a series of upgrade options which will be compatable with your computer. I have used them a number of times and have been impressed with the service.
A memory upggrade is often the cheapest and most effective performance upgrade that you can make to you computer.