Oh, I disagree with that. Yes there will always be more powerful computers, but that doesn't mean people will need that extra power. In fact, we've already reached that limit for most people. Most people do word processing, some simple photo touch ups, mp3 playing, spreadsheets, and things like that. The computer industry effective reached the threshold for all the power that is necessary to do all of those tasks in a desktop with essentially instantaneous results.
Even at the expert level, tasks like word processing will be just as instantaneous on a well maintained 3 year old computer as they are on a brand new modern machine. As newer machines get faster and faster, they will eventually reach the threshold where even most expert photo users will have a machine that almost instantaneously does what they want. We aren't at that point yet, but certainly within 10 years we will be.
But that's not to say people will stop wanting new computers. People will want new computers for the new features and capabilities that they add, a big example of which is portability. Laptops are taking up more and more of the computer market (over 50% of the new computers sold iirc), not because they're faster or more powerful than desktops, but because they offer a new functionality - portability.
So my prediction: within 10 years you'll have desktop computers that are fast and powerful enough to run basically every standard expert photo function at near instantaneous speed. But within 10 years we'll have brand new uses for our computers which will continue driving the need for more powerful machines, such as 3D photography or some other such thing.
p.s. That quote about Gates said 640k of ram, and it happens to be a completely untrue story.
http://www.wired.com/politics/law/news/1997/01/1484