A customer recently needed help changing thier Exchange Server installation from one server for database and SMTP to front-end back-end installation architecture. These docs can help walk you through this process:

Configuring Exchange Server 2003 for Client Access - Upgrading Front-End and Back-End Servers
http://www.microsoft.com/technet/prodtechnol/exchange/guides/Ex2k3DepGuide/ee9743f1-0c5c-462f-b322-aa0994ef1f5e.mspx

Deploying the Exchange Server Architecture - Configuring a Front-End Server
http://www.microsoft.com/technet/prodtechnol/exchange/guides/Ex2k3DepGuide/d511119d-dad2-4a12-9819-9df64e8696ae.mspx

Using Microsoft Exchange 2000 Front-End Servers (works for 2000 and 2003)
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=AFAD8426-572E-40F8-99DA-EB7198F374C4

Planning an Exchange Server 2003 Messaging System
http://www.microsoft.com/technet/prodtechnol/exchange/2003/library/messsyst.mspx

Exchange Server 2003 Deployment Guide
http://www.microsoft.com/technet/prodtechnol/exchange/2003/library/depguide.mspx