If you're curious about future-proofing, perhaps we should look at how a computer becomes archaic in the first place. Because the truth is, future-proofing is only going to affect a small number of computer users in general.
First off, we must keep in mind that eventually, all computers (and technology) will become obsolete. Imagine trying to future-proof your Walkman in 1988, for instance. Yup, you could've added more battery power or a clearer sound, but you were never going to get your New Kids on the Block CD -- or MP3 -- to play on it. You can only go so far to adapt current technology to what the future holds. You could buy a really, really fast processor for your laptop, but what if anything without a touch screen is passé in three years?
Most importantly, what are you using your computer for? The irony of the situation is that people who probably want to future-proof their computers -- those using advanced software and resources -- are the least likely to have success preparing their computers for future use. If you're primarily using your PC to type up Word docs or to connect to the Internet so you can answer your cousin's e-mail, you're in luck. Your computer will probably last a lot farther into the future because you aren't requiring much from it, other than what was already on the market in 1996.
But those of us who are heavily involved in the tech industry and crave cutting-edge games, software and processing are going to have a little more trouble. Those things are rapidly changing in the market and not just because they're getting better or more progressive. In the case of RAM, for instance, it's becoming a lot cheaper to get more memory. If you had future-proofed your computer with a ton of RAM five years ago, you're probably be kicking yourself now, as the price of RAM has dropped significantly. As a result, paying for an upgrade now is a lot cheaper than having the biggest allowance you could get at the time.
That's another reason you should be keeping your eye on what the computing future looks like for your specific purposes instead of pondering future-proofing. Think about what you want for now and you're more likely to be able to make strategic, value-driven upgrades in the future.