I agree with Andrew, in that Mars is the better starter planet. Although it has no atmosphere, I believe that it would be easier to transport extra water, and CO2 there and establish a greenhouse. The atmosphere of Venus is made worse by its slow retrograde(backward) rotation. A day on venus is slightly longer than its rotation. 280ish days of day heats it up so much that when the darkside cools it cannot cool enough. The slow rate at which it revolves also makes it immpossible for a magnetic field to be strong enough to diflect the Solar winds. The Gamma rays cannot be blocked and would kill all plant life that we attepmted to start, given the solar blocker worked.

In all honesty I believe that the next stepping stone for mankind will be one of the moons of Jupiter. Europa has abundant liquid water to harvest and the Sun will only get larger as it approaches the end of its life cycle.