Windows Server 2008 Hardware Requirements

Windows Server 2008 Hardware Requirements:
Processor
Minimum: 1 GHz (x86 processor) or 1.4 GHz (x64 processor)

Note: An Intel Itanium 2 processor is required for Windows Server 2008 for Itanium-Based Systems

Memory
Minimum: 512 MB RAM

Maximum (32-bit systems): 4 GB (Standard) or 64 GB (Enterprise and Datacenter)

Maximum (64-bit systems): 32 GB (Standard) or 2 TB (Enterprise, Datacenter, and Itanium-Based Systems)

Disk Space Requirements
Minimum (32-bit systems): 20 GB or greater

Minimum (64-bit systems): 32 GB or greater

Note: Computers with more than 16 GB of RAM will require more disk space for paging, hibernation, and dump files

Drive
DVD-ROM drive

Display
Super VGA (800 × 600) or higher resolution monitor

Other
Keyboard and Microsoft Mouse or compatible pointing device

Microsoft Assesment and Planning Tool:
Download the Microsoft Assessment and Planning tool evaluates existing servers and generate a migration report for Windows Server 2008, Windows Vista, Hyper-V, SQL Server 2008, and eventually Windows 7.

Source: http://www.microsoft.com/windowsserver2008/en/us/system-requirements.aspx

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Windows Small Business Server 2008 Standard and Premium Editions Hardware Requirements

Processor:
Recommended:
2 GHz (x64 processor) or faster for single core
1.5 GHz or faster for multi-core

Memory:
Minimum: 4 GB RAM
Maximum: 32 GB

Available Disk Space:
Minimum: 60 GB

Fax Modem:
Fax Services require fax modem

Network Card:
Ethernet network interface card from the Windows Server Catalog

Internet:
Some Internet functionality may require Internet access (fees may apply) and a Windows Live ID Account.

Supported Client Operating Systems:
Microsoft Windows XP Professional Edition SP2 or later; Windows Vista Business, Enterprise, and Ultimate; Windows Mobile 5.0 or later

Additional requirements:
* DVD-ROM drive
* Super-VGA (800 × 600) or higher-resolution monitor
* Keyboard and Microsoft Mouse or compatible pointing device
* Internet access (fees may apply)

NOTE: Windows Small Business Server 2008 supports a maximum of 75 users or devices.

Source: http://www.microsoft.com/sbs/en/us/system-requirements.aspx
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Windows Essential Business Server 2008 Standard Edition and Premium Edition Hardware Requirements

Processors
Systems with a Single Core CPU – x64 with a 2.5 GHz minimum clock speed

Systems with a Multi Core CPU – x64 with a 1.5 GHz minimum clock speed

Systems with Multiple Physical CPUs – x64 with a 1.5 GHz minimum clock speed

RAM
Management Server – 4 GB of RAM

Security Server – 2 GB of RAM

Messaging Server – 4 GB of RAM

Hard Disk Drives
Operating System Drive Partition – Minimum of 45 GB of free disk space

Application Data Drive Partition – Minimum of 50 GB of free disk space

DVD-ROM – Must be bootable from the system BIOS or a peripheral connection such as a USB port.

Network Interface Card (NIC) – One network adapter that supports a 10 Mbps or greater link speed. Two network adapters are required for the Security Server.

NOTE: Windows Essential Business Server 2008 Standard Edition is only available as a 64-bit operating system. Windows Essential Business Server 2008 Premium Edition adds a fourth server running either 32-bit or 64-bit versions of the operating system.

Source: http://www.microsoft.com/ebs/en/us/system-requirements.aspx