(See install new memory) (soon)
No more than necessary
Many people associate more memory to increasing the performances of a computer. That is a myth.. too popular unfortunately. In fact adding more memory does improve performances inside the following rule, there will be a benefit until you have enough memory for your use.
This is due to one thing : softwares need memory during operation and when they do not have enough, they have to compensate by using the hard drive which is much slower for that purpose. If your computer has enough memory, this phenomenon never occurs and adding memory is a waste of time and money.
For usual needs (email, web browsing, photos, music), the minimum we recommand is.
For Windows XP : 512 Mb
For Windows Vista : 1024Mb
For Windows 7 : 1024Mb
For Ubuntu : 512 Mb
For Mac : 1024 Mb
If you want to be comfortable in any situation, double the recommendation, but no need to go beyond! Unless you start to play with photo editing, video or heavy 3D games, then now adding more memory would be a winner choice for you. You will notice though that most of the time, computers have enough memory and there are far more effective ways to increase the performances of a PC