According to Microsoft:

A Windows Vista Premium Ready PC includes at least:

• 1 GHz 32-bit (x86) or 64-bit (x64) processor¹).
• 1 GB of system memory.
• Support for DirectX 9 graphics with a WDDM driver, 128 MB of graphics memory (minimum)², Pixel Shader 2.0 and 32 bits per pixel.
• 40 GB of hard drive capacity with 15 GB free space.
• DVD-ROM Drive³.
• Audio output capability.
• Internet access capability.