Solar thermal can provide "baseload" power (if you must obsess about this nebulous idea from an age when much of the supply was static and demand wasn't price aware).

There are already lots of countries that get far more than 20% of their power from renewables - the question is who will be first, and when will we get there.

Al Gore is apparently proposing that the US make the shift by 2020:

http://www.dailykos.com/story/2008/7/17/144426/316/203/553055

Probably a little ambitious, but I'd hope that by 2040 - 2050 this is the reality - plenty of time to depreciate the existing stock of generators and replace them with solar/wind etc.