Well, I think I would just go with the obvious.
http://www.christianforums.com/ has over 131,000 members.
No reason why you couldn't make a forum for atheists like
http://www.atheistnetwork.com/ which has over 3000 members.
I would invest in a vbulletin license, hire someone to make a basic vbulletin theme (skin), and start advertising.