First off, lets make sure that the difference between RAM and Hard Drive space is understood.

RAM is used as "temporary storage" by your computer. The amount of RAM you have determines the amount of programs and services the computer can have open and running.

Hard Drive space is physical storage space. This amount determines how many documents, pictures, music, videos, etc you can store on your hard drive.

So what happens when your computer runs out of memory (RAM)?

When your computer runs out of memory, your computer will start to use the hard drive space for "Virtual Memory". The problem with this is that your hard drive read/write speeds are much slower than the RAM read/write speeds. You will notice your hard drive activity LED constantly flashing, and your computer will start to run slow. The more "Virtual Memory" used, the slower your computer will run.

The simple solution to this is to add more RAM. However, there are several different types of RAM depending on if the age of the machine, and if it is a laptop or a desktop.

The simplest way to find out which type of RAM your machine requires is to go to http://www.crucial.com and use their Crucial Memory Advisor Tool.