A dozen or so years ago, RAM was relatively pricey and you couldn’t simply buy it online. But as computers started getting faster and performing more intensive operations, cheap, readily available memory became a necessity. Conveniently, that also makes it one of the best, easiest upgrades you can make to your machine. After all, as your Mac’s hardware becomes dated, it may not run as fast as when you first took it out of the box. A little boost in memory could give it that new lease on life you’ve been hoping for -- and save you from having to purchase a whole new system just yet.
If you're having some serious performance issues with your Mac -- say, applications seem to crash randomly when you start them up, your Mac is incessantly freezing, or some software just hangs without really loading -- you'll want to check to see that the RAM is really the issue and you're not simply in need of a software update. Use Memtest to run a Unix command that checks on your bad RAM.