Here is what I have estimates on: There exists hardware capable of 60 gigahashes per second. Over the next few months, X amount of these will be mining bitcoins.
Difficulty is recalculated every 2016 blocks. X * 60,000 megahashes = Y terahashes to total mining.
At current difficulty it will take a machine calculating at 60 gigahashes about 3.6 days to solve 1 block. With just 700 of these machines each solving 1 block every 3.6 days, difficulty will be at Z within a week, where difficulty is then recalculated to even out the time period of solving blocks (if I understand correctly).
The variable I have is about what the implications are of the algorithm for difficulty. Is it intended for there to only be a finite number of blocks solved in a certain timeframe?