Well, from a purely consequentialist standpoint, it would very likely be much more cost-efficient to donate the money you could use to make the data centre green to charities.

Then again, I'm plenty cynical to realize that going green is most often a PR-move, in which case donating to charities doesn't quite cut it. Nevertheless, if nature is the priority, a good charity (there are some bad, or at least inefficient ones, too) is best.