What is RAM and why do I need it?
Adding random access memory (RAM) to your system is the cheapest and most effective way to make it run faster. You can think of RAM as an incredibly fast hard drive that stores information temporarily instead of permanently. When you start a program it is loaded from the hard drive into RAM. When a program is running in RAM it can run hundreds to thousands of times faster than it can if run directly from the hard drive. The problem is that the capacity of a standard hard drive is many times the size of a computer's RAM size, meaning it is possible to load so many programs that the RAM can no longer hold them. When that happens, your computer's virtual memory kicks in, and that's bad.
Virtual memory is simply your hard drive trying to act like a RAM chip. Since the hard drive is so much slower than real memory, programs stutter and sometimes crash when the hard drive has to do a job it was never designed for. There are only two solutions to this problem: close some programs until virtual memory is no longer needed, or add more physical memory. If you can afford it (and current memory prices are low enough that practically anyone should be able to), the latter solution is always preferable.
With the addition of more RAM, you'll be able to keep more programs open at once, speed up program launches, and experience fewer system crashes. Plus, it's a pretty easy upgrade to make, requiring little technical expertise.