You don't have to upgrade that much at all.
That's because games are now made for consoles. As long as your computer is as good as a console from four years ago you'll be fine. Looking at the PS4 hardware so far I won't even have to upgrade anything for the next generation of games.
I've been going old school for a while so now I'm playing Maniac Mansion 1 and after that it will be Day of the Tentacle. Hell yeah!