When a site hosts its own SSL server (ie, you host the credit card form on your domain,
https://secure.yourdomain.com) I expect an increase in conversion rate because of the improved usability. Surfer never leaves your site, you control signup and surfer flow. Bingo, higher conversions.
How much more? 10%? 20%?
Verisign says there's a 32% increase in conversion when you host the signup form.