I think enough is enough when you can do whatever you want without too much hassle.
Way back when, in the time of the first pentiums and the 486, we got my in-laws a computer. It was a 286, some small games, WP5.1 on it and that's it. At some point, my mother-in-law decided she wanted a scanner, so she bought one. But, the 286 couldn't cope with that very well and we got them a fast 486 (lots of memory, at least considering the PC
).
Everything ran perfectly again until my mother-in-law wanted that printer. She started to use the PC to make pictures and such. So, that 486 didn't cope anymore and we gave them one of our old pentiums (we never bought that 486, we just gave them one of our old ones).
In the end, they always were one stap behind the "current" level. But they used the PC from start to finish.
In contrast, some friends of theirs bought that Pentium 90 soon after we had given that 286. They bought a printer and scanner. And never used it.
I use my PC for gaming mostly, and some programming on the side. So, I like my PC to be decently up-to-date. Not that I'll buy a new one every year though...