With today's desktop computers, we've pretty much hit "good enough" processor speeds at 3GHz. We're still squeezing more power out of RAM, multicore architectures, hard disk space and video cards, though. Personally, I wonder at what point we'll get to "good enough" in each of those components, and which one will top out first. Will people be perfectly happy with a 5TB hard drive, and anything larger is a waste, or will we never hit that mark? People are pretty bad at cleaning up after themselves and many consider a full hard drive the signal that it's time to buy a new PC, so it's entirely possible that there is no point where most people will have enough disk space for the indefinite future.
Another reason we may never hit "good enough" is that we always think of bigger, better and more powerful things to do with our computers. A 3GHz processor might be good enough for now, but soon enough people will be wondering why we can't do real-time video processing or some other task, so we'll need more powerful machines again. "Good enough" is relative.
Mokalus of Borg
PS - When you push these boundaries, you get bigger boundaries.
PPS - Eventually.