This is one most asked question in computer forums. Well the answer differs from individual to individual.  The programs you use determine if you need computer memory upgrade. The required amount of memory or ram also differs on different operating systems. If you only surf the internet and watch movies 2 gb of ram will give you nice and smooth experience. But if you are into image editing or video editing then you must add more ram. Always go for the same brand and same specifications of ram as installed in your PC. Different modules in the same computer can create instability and cause errors.