So, in these days of multi-core systems with multiple gigabytes and hundreds of gigabytes or multiple terabytes of disk, how much computer is enough?
Enough for what? If you are interested in modern games - look at their "recommended requirements" section (be ready after 3 years games will want more, even if they will be same as on $5 calculators - it's marketing, nothing personal). If to use office stuff and Internet, then look at requirements of these applications - better older versions as in common they have what you need at home but have much less requirements.
For home I'd buy PC from $300 and a monitor from same cost. To buy new hardware cheaper is just a problem. If you want "modern" PC - it's from $1000, where every hardware is close to avarage prices. For office stuff and a "little of gaming" from $500. Get a pricelist in local computer shop and fit hardware to preferable cost.
You may also find usefull articles at review sites as tomshardware, - there are many similar, on different languages.
We may not be quite there yet, but Windows 7 and up can address 16 exabytes of RAM
Better look how much modern CPUs support.
I think we will probably make some sort of exaflop supercomputer system in the next 10 years
There is a tendency of reducing improvement of speed for home computers. After 10 years I suppose home PC will be more a terminals. Your software and data is on a "exaflop supercomputer" and you pay every month for a possibility to use it, like today you pay for electricity. No "piracy" and total control of Big Brother. Even if technology would allow "exaflop supercomputers" in every home, then people at power would prefer to hold it. I think at middle of 2000s such decision was done, as I don't see the progress of home electronics wich was befor, but I see as nets became cheaper and accessible in every coner, see as smartphones wich easily may be used for spying became spreaded, see as cameras were set up at streets, see as medias lie with more impudence, as educational level of people degrades in so called "developed states". These are signs of establishing rude dictatorship where mass people should not to have "super computer" at home on principle.
But I think we've crossed the thresh-hold of "good enough" a while back
There is just no serious progress. That's why such impression. We have not much to compare with.