Two Servers in two different locations - do they need to be running Enterprise?

If they are identical then you are talking about clustering. Clustering is only done with 2003 Server Enterprise.

A geo cluster is going to cost you a fortune.  What you could use instead is something like doubletake or neverfail, which will work on Standard Exchange.

What should really determine your choice of version is the storage size, Standard is limited to 75 GB

In addition, if your server needs to run more than 4GB of ram or you need to be able to use hotswap memory then there is a few more reasons for using 2003 Server Enterprise as the server OS.

http://technet2.microsoft.com/windowsserver/en/library/81999f39-41e9-4388-8d7d-7430ec4cc4221033.mspx?mfr=true

As Kieran mentioned, Exchange 2003 Standard would be used if you expect to use a 75GB or smaller database (after applying SP2). Exchange 2003 Enterprise would be used for larger databases.

http://support.microsoft.com/kb/822440

Of course you want to test everything before committing. Test with the Microsoft VHD program if you don't have TechNet. http://technet.microsoft.com/en-us/bb738372.aspx

We need more information as to what you are trying to achieve. Just having two servers in separate locations would simply require Standard edition - then you could use a third-party tool, or even just DFS to replicate data between the two.

Thanks for the responses, the question is a bit vague.

As our clients business is growing we are moving away from SBS 2003, We are going to setup two locations (Sites?), Site 1 will have the Primary Domain Controller with a Secondary Domain Controller in Site 2 and Exchange installed on both Controllers.  Our question is which version do we need with Server 2003 and Exchange 2003, Enterprise or Standard?

Each Controller/Site will have DNS running and Site 2 will be also be a Global Catalogue Server for redundancy. we also intend Publishing a DFS root for company Data.

Is there any advantage in creating two Sites? (AD), opossed to both Controllers being in the same site

What is the process for installing Exchange in this environment,?

Our aim is to provide redundancy between Sites, e.g If Site 1 goes down both Sites can still function utilising the Controller in Site 2 and Vica Versa.

we intend on setting up a test platform to optimise the installation onsite, any assistance on the installation/Configuration would be greatly appreciated.