How many servers needed?

Hello,

Does it possible to host and deploy Exchange server 2007 on the server host domain controller, active directory and DNS ?

I would like to open up the Exchange server to the Internet so it can host the public E-Mail !

My question is: How many servers do I need to deploy Exchange 2007 and achieve our desired functionality? Can I set up all server roles on a single server or do I need two or more servers?

I have only one server, Windows Server 2008 DVD and Exchange server 2007

Thanks in advance for any help on this topic!
WP