I'd say that a growth of total computing power to between 10,000x and 10,000,000x of the current one in 20-30 years, followed by stagnation or perhaps a slow growth, seems reasonable

From Wikipedia:

Although this trend has continued for more than half a century, Moore's law should be considered an observation or conjecture and not a physical or natural law. Sources in 2005 expected it to continue until at least 2015 or 2020.[note 1][11] However, the 2010 update to the International Technology Roadmap for Semiconductors predicts that growth will slow at the end of 2013,[12] when transistor counts and densities are to double only every three years.

It's already happening.

Current process size is ~22nm, silicon lattice size is ~0.5nm . Something around 5..10 nm is the limit for photolithography, and we don't have any other methods of bulk manufacturing in sight. The problem with individual atoms is that you can't place them in bulk because of the stochastic nature of the interactions.