Personally, I'd want to know what charity I'm donating to before making the commitment. You should just choose one and go from there.
I've been burned before with online forum organized charity events so I'd also want to see some controls in place to ensure funds received actually go to the charity. Something as simple as a picture handing over a cheque for the amount received or a scan of the receipt. Call me paranoid, and I'm not saying you're out to make a buck, but I've seen the scam happen.
You could even get the charity involved and have them collect the money directly. I've done some in the past with groups like MADD where we had a MADD representative attend and as a bonus they supplied small prizes, stickers, ribbons etc...