If a user wants to make a donation, he has to fill in a minimum amount of data. Name, address and CC card. This is information we can store. If we ask for an email (OPTIONAL) it should not be too much of a hassle for the user. Password will be send to his email address once he hits the donate buttom. Log him into the system, and set a cookie unless his email bounces :).
If the user returns and wants to make a new donation, he should be shown the values already filled in (name, address, amount, cc info), thanks to the cookie. Once the donation is made, he should recieve an email about it (again, if he supplied a valid email only).
As for anonymous donations. Is it legal? Or more precise, is it legal to accept them? Just wondering about a lot of skandals we had here with anonymous donations as this can be used for money laundring. If it is, make sure you support it, one way or another, though I personally would never like to forfeit my tax benefit that comes with donations :).