There's - well, I guess it's a little bit early to call it a movement, but a wholly admirable suggestion from fandom that we do our bit and try to match his donation. It has a name - Match It For Pratchett - and there are even teeshirts.
But the most direct thing you can do is make a donation here, which I'm going to be doing. Apparently you can leave a note saying you're doing it to Match It For Pratchett.
Apart from being mordantly funny (the line about the dead mole made me laugh so much people in the office became concerned) what Terry says in his speech is right. Alzheimer's has always been a sort of shameful secret and, while no one in their right mind would grudge cancer research their funding, it's nothing short of a scandal that Alzheimer's research gets by on a fraction of the cancer budget.