http://www.microsoft.com/windowsvista/getready/capable.mspx

A Windows Vista Capable PC includes at least:

• A modern processor (at least 800MHz¹).
• 512 MB of system memory.
• A graphics processor that is DirectX 9 capable.

Windows Vista Premium Ready PCs
To get an even better Windows Vista experience, including the Windows Aero user experience, ask for a Windows Vista Capable PC that is designated Premium Ready, or choose a PC that meets or exceeds the Premium Ready requirements described below. Features available in specific premium editions of Windows Vista, such as the ability to watch and record live TV, may require additional hardware.

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.