Besides installing an SSD to replace a mechanical hard drive, RAM upgrades are one of the more significant and beneficial upgrades you can do for a PC system. When it comes to Apple's systems, however, RAM upgrades from Apple itself have been insanely expensive, so my recommendation has always been, unless the RAM is soldered to the motherboard (as it is with the MacBook Air and Retina MacBook Pro), to purchase only the base amount of RAM and then upgrade it yourself later on.
Such was the case with my most recent Mac, a 17-inch 2011 MacBook Pro that I purchased with a mere 4GB of RAM. Instead, I opted to purchase a third-party 16GB Mushkin RAM upgrade for around $130, getting far more memory at less expense.
The RAM arrived and installed without a hitch, and passed Apple's Hardware Test suite and other RAM testing routines with flying colors. After performing such an upgrade, you are left with the older RAM modules that are essentially useless to the system. In some cases manufacturers offer a trade-in program for your old memory, or you may be tempted to sell the memory on eBay, give it away, or simply toss it. My recommendation is to at least keep your old RAM modules, but also keep your new RAM's packaging and receipts.
Even though in most cases RAM upgrades that pass hardware testing will work just fine for the lifetime of the computer, there are occasions when a problem can be introduced that will escalate over time and result in complete system failure.
It started with a crash
After having the Mushkin RAM in my system for nine months, often using it to its full capacity, I ran into a single kernel panic a couple of weeks ago that, upon restarting, suggested a problem with some third-party kernel extensions for VirtualBox in the system. Since problems with kernel extensions are often the cause for such crashes, I uninstalled VirtualBox and removed these and other extensions that weren't in use. For awhile the problem seemed better, but a few days later the system took a downward spiral. While it would boot just fine, occasionally it would switch off and restart, or hang upon waking from sleep and require a forced reboot. These behaviors became more frequent, but were still somewhat tolerable.
The final stage of the problem began when the system automatically restarted but then would not boot, and instead output three loud beeps (with corresponding flashes of the of the power LED) that repeated indefinitely. These signals are Apple's indicator that something is very wrong with the hardware of the system, the three beeps translating to bad RAM that cannot be used. Forcing the system to power off and back on resulted in it booting again; however, a little later it crashed again, this time remaining at the hardware failure tones. It was dead.
It felt like the system had just sipped from the wrong cup in "Indiana Jones and the Last Crusade," and was withering before my eyes.
Granted, the three beeps indicated a problem with the RAM itself, but the most reliable way to test this is to install working RAM modules and see if the system boots without issue. Of course not having Apple's "AppleCare" protection plan made the potential costs of this issue a cause for significant worry. Not only might the repairs be expensive, but Apple no longer makes my beloved 17-inch MacBook Pro system, should the issue be costly enough to require a new computer -- doubtful, but understandably that's one of the scenarios going through one's mind in a situation like this.
Repercussions and remedy
Luckily, instead of getting rid of the original RAM after the upgrade, I had stored it in a safe place, which ended up being the savior of the day. Replacing the Mushkin RAM with the original memory resulted in a solid boot and a stable system that has been running fine ever since. The fix took minutes instead of hours waiting for local stores to open (and likely sell RAM at a premium), or to take the system to Apple. To me, option of getting back up and running was worth far more than the price of the trade-in. For this reason, I recommend that if you have upgraded your RAM, to keep your old modules as a backup just in case something goes awry.
Also, when you purchase new RAM it comes in molded plastic packaging. This is ideal for securely storing your old modules, ensuring they stay dust-free and are not subject to physical damage.
Finally, should you find yourself in this predicament, there is a light at the end of the tunnel. Most RAM manufacturers guarantee their products for life, which means that if it happens to you, you'll likely be able to arrange a replacement at minimal cost. You'll probably need your original proof of purchase for such an exchange, so print it out and store it with the old RAM so you have a one-stop fall-back-and-replacement package available.