Given am Exchange Server 2003 running on a dedicated server on a LAN protected by a Linux in DMZ, Microsoft says that if you upgrade, you should install two different servers (meaning two boxes, two licenses of Windows Server and two installations of Exchange) with different Exchange "server role".
Exchange is installed in a safe LAN, there is a Linux relay in DMZ that feeds messages to Exchanges and gets from it the messaged to be delivered on the Net (smart relay).
The mail traffic is about 2000 Internet messages/day and probably another 2000 msg/day sent by local users within the organization. The servers hosts 200 users.
We just deployed Exchange 2010 with single server & configured the hub role to be Internet facing as per this info: link text
No you don't - your Linux relay is performing the main function of the Exchange Edge server.
Sure an Exchange server would have more functionality (but not necessarily performance), but it is pretty easy to add Spam Assassin/ClamAV... to a Linux server to replicate some of its features.