I don't see how funding research to fight a disease is "rewarding" people. Is increased funding for a cancer a "reward" for those with terminal lung cancer?
Fighting AIDS will help in all other aspects of medical research, because really when you talk about finding a cure for AIDS, you are talking about finding a cure for viruses in general (which, for the most part, we cannot do yet).
Also, you are assuming that Bush will just write a check to dictators in Africa. I believe it will not be distributed in that manner.
If he does it through some UN or leftist NGOs it essentially ends up the same way.