Whenever we run an application or tell the computer to perform a certain function, the information from that command is loaded onto memory. With that memory on hand, your desktop can run more efficiently. But when your computer has too many tasks to perform and not enough means with which to do everything, you're putting strain on your desktop's processing. It's sort of like asking a person who's spent a couple of weeks training for a 5K race to run an entire marathon -- the runner wouldn't have the strength to run such a long distance in a reasonable amount of time since he or she would be woefully unprepared.
So if your desktop is suffering from too much workload, how much RAM do you actually need? Before you go out to find the RAM module on the shelf, you need to ask yourself what kind of work you normally do on your desktop. If you use your desktop for just your bare-bones basics like e-mail and word processing, your computer won't need too much RAM. Somewhere between 384 to 512 MB of RAM should be sufficient. But you should keep in mind that even if your desktop computer is relatively new, some systems don't come with enough memory to operate even the most basic tasks.
The more processing you require your computer to perform, however, the more RAM you'll need. If you use your desktop for work in a home office and you often use several different types of programs simultaneously, including e-mail, Web surfing, word processing, spreadsheets and software for presentations and illustrations, you desktop will probably require the upper end, maybe even as much as 1 GB of RAM. Serious gamers who use up a lot of graphics power need between 1 and 2 GB of RAM, while professionals who use a lot of 3-D modeling software should probably have 2 GB or more.
If you're not entirely sure what kind of RAM your desktop you'll need, there are Web sites like Crucial.com that are dedicated entirely to memory performance and allow you to select the type of desktop you own and search for the most compatible type of RAM for your system.