[yor_on]

For the Basic Windows 7

    * 1 GHz 32-bit (x86) or 64-bit (x64) processor
    * 512 MB of system memory
    * 20 GB hard drive with at least 15 GB of available space
    * Support for DirectX 9 graphics and 32 MB of graphics memory
    * DVD-ROM drive
    * Audio Output
    * Internet access (fees may apply)

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For the Premium Windows 7
    * 1 GHz 32-bit (x86) or 64-bit (x64) processor
    * 1 GB of system memory
    * 40 GB hard drive with at least 15 GB of available space
    * Support for DirectX 9 graphics with:
          o WDDM Driver
          o 128 MB of graphics memory (minimum)
          o Pixel Shader 2.0 in hardware
          o 32 bits per pixel
    * DVD-ROM drive
    * Audio Output
    * Internet access (fees may apply)

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Here's What You Need to Use Windows XP Professional

PC with 300 megahertz or higher processor clock speed recommended; 233 MHz minimum required (single or dual processor system);* Intel Pentium/Celeron family, or AMD K6/Athlon/Duron family, or compatible processor recommended
•   

128 megabytes (MB) of RAM or higher recommended (64 MB minimum supported; may limit performance and some features)

1.5 gigabytes (GB) of available hard disk space*
Super VGA (800 x 600) or higher-resolution video adapter and monitor
CD-ROM or DVD drive
Keyboard and Microsoft Mouse or compatible pointing device
and a netcard and a ISP
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and for Linux?
Depends on what you want.
Any computer will be able to run some distribution out there.
from a http://en.wikipedia.org/wiki/Intel_80286 to the latest models.

[LeeE]

Lol - 512MB ram for Windows 7 and 128MB for XP - they're having a giraffe.