Windows 2008 Server Editions

Windows Server 2008 and Windows Server 2008 R2 is a server operating system produced by Microsoft. It was released to manufacturing on July 22, 2009 and launched on October 22, 2009. According to the Windows Server Team blog.
There are seven editions: Foundation, Standard, Enterprise, Datacenter, Web, HPC Server, Itanium and Windows Storage Server 2008 R2(Essentials Edition).

Processor

1.4 GHz x86-64 or Itanium 2 processor

Memory

Minimum:512 MB RAM (may limit performance and some features)

Recommended:2 GB RAM

Maximum: 8 GB RAM (Foundation), 32 GB RAM (Standard), or 2 TB RAM (Enterprise, Datacenter and Itanium-Based Systems)

Display

Super VGA (800 x 600) or higher

Disk Space Requirements

Minimum (editions higher than Foundation): 32 GB or more

Minimum (Foundation edition) 10 GB or more.

Computers with more than 16 GB of RAM require more disk space for paging and dump files.

Other

DVD drive, keyboard and mouse, Internet access (Optional, but required for updates and online activation)

 Edition Comparison Chart:

click  picture to Enlarge