RAM or Random Access Memory is a critical component for a computer to run properly. Having not enough RAM can render a computer useless over time due to too many programs or applications being used at the same time. The more applications a person opens, the more RAM is needed to run those applications. Today, most computer manufacturers sell new computers with at least 512MB of RAM. Compared to a few years ago, the minimum amount was 256MB. And before that, the minimum amount was 128MB or even 64MB. The increase is due to the complexity of the software and Operating systems that are being installed on the computer. For example, Windows XP has a bare minimum RAM requirement of 64MB to operate. But according to Microsoft, it is recommended to have 128MB to "increase performance". As for myself currently working in the PC support industry, you really need at least 256-512MB of RAM to get any type of performance out of Windows XP. If not, as soon as you start installing any type of games or database intensive software, the computer will slow down dramatically. Of the people that I support either at work or my personal clients, one of the main issues is not having enough RAM installed.
As another example, if an application uses 128MB of RAM, and the PC only has 128MB installed, that application will do one of two things:
1) Not even launch
2) Will launch, but then the PC will become unstable and eventually lock up.
When a PC runs of out RAM (or its all being used) then the PC will have to find RAM from somewhere else. Usually this means it converts some hard drive space into a term called Virtual RAM. This slows the computer down even more since its using "fake" RAM to try to help the computer run. Then the issue of available hard drive space becomes a factor. If you are low on hard drive space and the PC is attempting to convert some of the space into Virtual RAM, the PC will become even more unstable. You do not want to get to this point because again, the PC will eventually crash or lock up.
Bottom line, you can never have too much RAM installed in your PC. If you can afford it, I recommend purchasing the most amount of RAM that will fit on your PC. Usually these days, most newer PCs will hold 4GB of RAM. And of course, most PCs now come with Windows Vista pre-installed. Windows Vista will not even run with less than 1GB of RAM! So if you plan on upgrading your PC, spend some extra money to increase RAM as far as it will go.