How to replace "checkout directly" with create a new account and continue checking out?

How to replace "checkout directly" with "create a new account" sign up form and continue checking out?

http://screencast.com/t/e2vscWRnx4m

Once a new user fill out the sign up form, and confirm email, he/she can use the left side login form to continue checking out.

Thank you for reading my question :slight_smile: