People always overestimate how fast technology is moving. It moves fast, yes, but not THAT fast. Two anecdotes:
Two years ago NY Times said in an editorial that it was time to get rid of your Desktop PC and digital camera. Well, guess what? I still use both of them, and if need be, I'd buy one again feeling comfortably confident that my desktop PC would remain to be useful for at least 4-5 more years.
Same goes with my e-reader. 2-3 years ago, everyone was telling me to blow $500 on an ipad instead of $79 on a kindle, because the tablets were supposed to replace and make obsolete e-readers anyway. Well, guess what? I used by my e-reader happily for 2 years, and only recently upgraded to a $200 tablet. Meanwhile, everyone going around saying that "the future is here" is on their second or third $500 ipad in 3 years.
So, the moral of the story is this: Yes, the future is coming. But your current technology is likely still good now, even if it's a few years old, and it will remain to be good for a few more years. Anyone who tells you not to buy or build a PC now because of what the future will bring is either anticipating way too big and fast of a changed, or brainwashed by some tablet marketing scheme.