Actually it is possible, but it requires an actor to speak around 200 base words, and then a certain program will create a "voice profile" and will be able to generate a voice and speak any programmed words typed into the database. I saw a tv show about the program a while back. The problem is I'm pretty sure the technology is extremely expensive.
Like a star trek computer voice?
Well, the technology is there but people like Stephen Hawking still sound rather robotic.
Maybe in a few years technology will have advanced to the point a voice synthesiser sounds natural.
Im sure its theoretically possible, there are a finite number of phonemes in any language, and a finite way in which they can be combined.
On increasing pc power: It is a fallacy to think one can just double the number every few years.
There are limitations to the hardware, and then limitations to the very laws of physics.
Even if one could build some sort of supercomputer that uses holographic models (mathematical meaning, as in mandelbrot, not hollywood meaning) and a quantum processor one still would run into the fact that it will be limited by the speed of light.
There is no such thing as infinitely increasing processing power.