is usually to buy a SSD. I’m a big fan of Crucial MX300 275GB SSDs and their larger siblings. The 525GB model is a better deal. Price scales linearly from there. If you’re upgrading a desktop, these Icy Dock MB290SP-B 2.5 Inch SSD to 3.5 Inch brackets are excellent. If you have less than 8GB of RAM, going straight to 16GB DDR3 RAM for your notebook (pair of 8GB) or 16GB DDR3 RAM for your desktop (pair of 8GB) makes sense, if your computer supports it. Crucial has an adviser tool on their website that will tell you what to buy. Newer machines have switched over to DDR4, older machines (2nd gen Intel Core series and older) won’t take 8GB DIMMs so you’ll want an 8GB kit (pair of 4GB). For DDR2 and earlier you might as well look on eBay for used parts if you bother upgrading at all.
If you upgrade to a SSD you’ll have to either clone your old HD to the SSD or clean install the operating system. I use Clonezilla, which isn’t the most user friendly and tends not to work when migrating from larger to smaller drives. Macrium Reflect is supposed to be good. I haven’t used the Acronis True Image HD that comes free with Crucial SSDs. You can download the Windows 10 installer from Microsoft. For Macs, Time Machine does an amazing job of restoring systems. Use it. If you want to clean install your Mac it’s fairly easy to make your own bootable macOS 10.12 Sierra USB install drive. iFixIt has toolkits and instructions for tearing apart Apple products. Older MacBook Pros are well worth upgrading with SSDs and RAM. Newer ones aren’t upgradable.