If you want to help Wikipedia, the best thing you can do is contribute to articles. Time and expertise is always going to be a more valuable donation than anything monentary.

I have different reasoning than the parent commenter. Why do you want to help fund Wikipedia? It is in no danger at all. Funding it even more lavishly probably won't help it achieve its goals more effectively; it's dependent (by design, and for good reason) on volunteer effort, which is the real bottleneck.