An article on Slate (link >>>) presents some calculations on the amount of energy required to terraform Mars and Venus for human habitation.
It turns out that Mars is a conceivable case with existing human knowledge. The endeavor would certainly be difficult and take a millennium or more, but it would apparently require less than 1 exajoule (10^18 J) to give Mars a thicker, though oxygen-free, atmosphere and a surface temperature more acceptable to humans. (A 100MW power plant produces 1EJ in 317 years.)
Terraforming Venus, on the hand, would instead require stellar levels of energy output (roughly 10^34 joule), thereby putting it in the realm of the inconceivable at our current level of understanding the universe. Its dreadful atmosphere seems to be the biggest hurdle.