It's been done before, with other ideas... and they were a fair bit more interesting than this one.
Sorry, just calling it like it is.
I think you should try something a bit more novel, like personalised 30 second video messages for anyone that donates more than $5.
For every dollar they spend over $5, they get another 10 seconds or something.
Then you pop the videos up with their names next to them, a little social-bookmark submit script like Addthis, etc.
Create a Youtube channel for it as well.
Don't forget to stipulate that you won't say certain sorts of things that could get you in trouble, like racial remarks or death threats.