Within the limits of who he is. Either he or Gates could put that money into supporting any of a number of polical causes - even given that you would not expect a billionare to be a revolutionary I can think of liberal causes they could support - Single Payer, Energy efficiency regulations, taxes on the rich (which Buffet - anyway - claims to support) . They could help a lot more people with those than giving direct charity of various sorts. Heck there are even stuff that is indirectly rather than directly political. Put a billion into a really large scale factory to produce solar cells - get their price down to $1 per watt. Build an inexpensive electric sedan like the Solectria Sunrise.
Of course I don't think anyone capable of becoming a billionare is likely to think like that. Why would ideas like these even occur to Gates or Buffet? But the idea that they are "doing the right thing" with their wealth does not seem right to me. They could worse with it no doubt, but they are not even beginning to put it where it would do the most.