In reality, I could care less if Africa were totally uninhabited in 50 years, but for the sake of argument here's what I would do. I would find one country on the continent that time forgot that was as free from corruption and internal strife as possible and begin building it up. I would concentrate all of my efforts on this one country, as opposed to spreading my resources thin trying to help out all of them. The goal would be to turn this one country into a beacon of civilization that would slowly spread and transform its neighbors into likenesses of itself. This one regional superpower could have a domino effect that could be the nucleus for a prosperous and transformed continent.

Such a process would probably take from decades to a century to complete, but considering that African civilization hasn't moved an inch in the past 10,000 years or so, I would consider it a major success. But then again, seeing how they've squandered the infrastructure left over from the colonial era, this may just be wishful thinking.