Topic: Considerations for a Crowdfunding application

I am looking to build a crowd funding app.  I have some reasonable experience with Rails, but wanted to ask those smarter than I, for advice getting started.

Essentially a project can be created with a value, Ex: $10,000.  Then other users can donate to that project.  $5, $100, whatever they choose to donate.

I think overall it makes pretty reasonable sense.  Are there any gems, plugins or frameworks in existence that could help the overall work load? 

I am thinking of using ActiveMerchant for the ecommerce side of things.