While I don't donate cash...
As I'm unemployed and have limited income as it is - I do donate my spare CPU time to the folding@home project http://folding.stanford.edu/ which it's main headling is "Our goal: to understand protein folding, misfolding, and related diseases" which covers Alzheimers and many more.
I've done this for years as an aid towards science - even before I learned my favorite author was diagnosed - and will continue to do so.
Hoping some (or most) will download and help this project.