Sure, Microsoft has its own table comparing the various Windows Server 2003 editions, but that version is incomplete, hard to read, and poorly organized, especially if you're interested in the features that are specific to the 64-bit editions of this product family. So here is my own comparative table, which I think you'll find to be much more readable.

NOTE: The response to this table has been incredible! Thanks to everyone who's written in with ideas, corrections, and support. --Paul

Key

Web - Windows Server 2003 Web Edition
Std - Windows Server 2003 Standard Edition
Ent - Windows Server 2003 Enterprise Edition
DC - Windows Server 2003 Datacenter Edition
Ent-64 - Windows Server 2003 Enterprise 64-Bit Edition
DC-64 - Windows Server 2003 Datacenter 64-Bit Edition

Comparing the Windows Server 2003 Editions


Processor support
Feature Web Std Ent DC Ent-64 DC-64
Compatible with 32-bit Intel x86 systems  Yes   Yes   Yes   Yes       
Compatible with 64-bit Itanium-based systems              Yes   Yes 
Maximum number of processors  2   4   8   32   8   64 
Hardware support
Feature Web Std Ent DC Ent-64 DC-64
Maximum amount of RAM  2 GB   4 GB   32 GB   64 GB   64 GB   512 GB 
Hot-add memory support        Yes   Yes       
Non-Uniform Memory Access (NUMA) support        Yes   Yes   Yes   Yes 
File and print server features
Feature Web Std Ent DC Ent-64 DC-64
Includes Distributed File System (DFS)  Yes   Yes   Yes   Yes   Yes   Yes 
Includes Encrypting File System (EFS)  Yes   Yes   Yes   Yes   Yes   Yes 
Includes Shadow Copy Restore (Requires 2000 or XP client)  Yes   Yes   Yes   Yes   Yes   Yes 
Supports removable and remote storage     Removable only   Yes   Yes   Yes   Yes 
Includes Fax service     Yes   Yes   Yes   Yes   Yes 
Includes Services for Macintosh     Yes   Yes   Yes   Yes   Yes 
Management features
Feature Web Std Ent DC Ent-64 DC-64
Includes IntelliMirror technologies  Partial   Yes   Yes   Yes   Yes   Yes 
Includes Group Policy Results  Partial   Yes   Yes   Yes   Yes   Yes 
Includes Windows Management Instrumentation (WMI) command line features  Yes   Yes   Yes   Yes   Yes   Yes 
Supports image-based remote installation  Yes   Yes   Yes   Yes   Yes   Yes 
Supports Remote Installation Services (RIS)     Yes   Yes   Yes   Yes   Yes 
Includes Windows System Resource Manager (WSRM)        Yes   Yes  Yes   Yes
Supports WSRM administrative GUI        Yes   Yes     
Active Directory features
Feature Web Std Ent DC Ent-64 DC-64
Can be an Active Directory member server  Yes   Yes   Yes   Yes   Yes   Yes 
Can be an Active Directory domain controller (DC)     Yes   Yes   Yes   Yes   Yes 
Metadirectory Services (MMS) support        Yes   Yes   Yes   Yes 
Security features
Feature Web Std Ent DC Ent-64 DC-64
Includes Internet Connection Firewall (ICF)     Yes   Yes      Yes    
Public Key Infrastructure (PKI), Certificate Services, and Smart Card support  Partial   Partial   Yes   Yes   Yes   Yes 
Terminal Server features
Feature Web Std Ent DC Ent-64 DC-64
Includes Terminal Server     Yes   Yes   Yes   Yes   Yes 
Includes Terminal Server Session Directory        Yes   Yes   Yes   Yes 
Can be administered through Remote Desktop  Yes   Yes   Yes   Yes   Yes   Yes 
Networking features
Feature Web Std Ent DC Ent-64 DC-64
Supports Virtual Private Networking (VPN)  Partial   Yes   Yes   Yes   Yes   Yes 
Maximum number of VPN clients  1   1000   Unlimited   Unlimited   Unlimited   Unlimited 
Includes Internet Authentication Service (IAS)     Yes   Yes   Yes   Yes   Yes 
Maximum number of RADIUS network access servers for IAS     50   Unlimited   Unlimited   Unlimited   Unlimited 
Maximum number of remote RADIUS groups for IAS     2   Unlimited   Unlimited   Unlimited   Unlimited 
Supports network bridging     Yes   Yes   Yes   Yes   Yes 
Includes Internet Connection Sharing (ICS)     Yes   Yes      Yes    
Supports IPv6  Yes   Yes   Yes   Yes   Yes   Yes 
Clustering features
Feature Web Std Ent DC Ent-64 DC-64
Includes Network Load Balancing (NLB)  Yes   Yes   Yes   Yes   Yes   Yes 
Includes Cluster Service        Yes   Yes   Yes   Yes 
Maximum number of cluster nodes        8   8   8   8 
Application Services features
Feature Web Std Ent DC Ent-64 DC-64
Includes .NET Framework  Yes   Yes   Yes  Yes      
Includes Internet Information Services (IIS) 6.0  Yes   Yes   Yes   Yes   Yes   Yes 
IIS is installed by default  Yes                
Includes ASP .NET  Yes   Yes   Yes  Yes      
Includes Enterprise UDDI Services     Yes   Yes   Yes   Yes   Yes 
Multimedia features
Feature Web Std Ent DC Ent-64 DC-64
Includes Windows Media Services 9 Series     Yes   Yes   Yes       

I'm interested in ensuring that this table remains the definitive guide to the different versions of Windows Server 2003. Please email me with any additions, comments, or corrections. Thanks!