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Windows Server 2003 was based on Windows XP and was released in four editions: Web, Standard, Enterprise and Datacenter. It also had derivative versions for clusters, storage and Microsoft’s Small Business Server. Important upgrades included integrating Internet Information Services (IIS), improvements to Active Directory (AD) and Group Policy (GP), and the migration to Automated System Recovery (ASR).
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If your software support 64 bits version, you can use a 64 bits server edition.
For the number of user your planing to your server a quad core processor should be a good option, and maybe around 4 - 8 Gb of RAM. Once again, it depends of the app you will use and their requirement.