realistically it isn't going to be anywhere near 20 years. If it was only the technology of memory/storage - I'd put it at about 5 years.

Currently an entry level PC has 4Gb RAM and 2Tb disk - a high end PC will have 16-32 Gb RAM and 4-10Tb disk.

One problem with RAM is that current processors can only support 32Gb RAM so to progress beyond that we need a new processor architecture - i.e. move from 64bit to 128 bit or start working on a *lot* more processor cores- The parallella project is certainly a start in that direction.

As for disk space we could easily be at 1000Tb capacity drives being mainstream in 2-3 years but these specs are more driven by gamer requirements than business and that is focussing more on GPUs at present so I would expect a plateuing of CPU/Memory/Disk over the next year before games start becoming CPU or Memory bound again at wich point I would expect the CPU/Memory requirements to start growin again.

I'll go with 1Tb ram being available 5-7 years, 1000Tb hard drive 3-4 years, 1000Tb SSD or equivalent 5-7 years