Finally, avoid the mistake of using an inadequate or ill-fitting heat sink with your CPU. If your new CPU is substantially faster than your old one, it probably creates more heat, too. So unless you're already using a high-performance heat sink, consider treating heat-sink replacement as part of the upgrade process. You don't have to go all out with an expensive, complicated liquid-cooling system, but if you're spending $300 on a new CPU, spending $30 to $50 on a high-quality heat sink to protect that investment (and the rest of your PC) makes sense.
Hard Drive Upgrades
Next to RAM, a hard drive is one of the easiest PC components to upgrade. Often, the most difficult part of the process is reaching all of the screws with your screwdriver. That's because many system cases open only on one side, or contain framing components that block access to the drive cage. Resist the temptation to take the easy route of screwing the drive in on only the more accessible side. An unevenly mounted hard drive is likely to wobble slightly in its bay, causing undesirable vibrations that can make your PC noisier than it should be and potentially shorten the drive's life.
Nearly all PC chassis are designed to give you access to both sides of the drive cage. In most instances, the cage itself is removable, so you can snap it out, screw your drive in properly, and then snap the cage back into place. Take the time to do this, and you'll probably reap the rewards of a quieter PC and greater longevity from your drive.
Another common mistake--even among experienced PC builders--is to use the wrong type of screws to mount your hard drive. This error usually isn't disastrous, because the difference in diameter between case screws and hard-drive screws is subtle. But 6-32 case screws are slightly thicker and have a wider thread spacing than M3 hard-drive screws, so using the wrong screw will mangle the screw holes on the drive, which may cause problems later if you ever need to remove and reinstall the drive.
Don't Ignore the Power Supply