Everything in this post is wrong, actually. But if you buy shit parts to build your desktop, you’ll have a shitty desktop.

Simple answer is at the motherboard level - you look at your motherboard’s future expansion capability and if you started with a good foundation you can do years of upgrades. Also your computer case needs to be big enough to fit extra stuff, full ATX motherboard size is great.

For example I have a VR gaming rig that runs VR games well on DDR3 RAM and a Sandy Bridge CPU, because it has a decent modern GPU and enough CPU cores + RAM.