perhaps "anonymous" isn't the right term there, since CC transactions can't be anonymous, but I do think the requirement that someone register at the site to be able to donate places an aditional barrier between the decision to donate and the money arriving in the destination account.
Also, if they register in order to donate and then forget their password, if they want to come back and donate again they'll need to make up a new bogus email address or go through a password retrieval process in order to be able to give you more money (again, a point where you're potentially going to lose them and their money).
I'd suggest that you record the donators info if they're a logged-in user, but also allow users who are not currently registered or logged in to donate...
