Microsoft updated their "Get Ready" web site with the system requirements for Windows Vista.  Machines who are labeled as Windows Vista Capable will be able to run the core experiences at a minimum, but more advanced features such as the Windows Aero interface might require more advanced hardware.

 A Windows Vista Capable PC must include at least:

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

For even better performance, including running the Windows Aero interface, PCs will have to meet or exceed the "Windows Vista Premium Ready" requirements. 

To be certified as a Windows Vista Premium Ready PC, it will need to have at least:

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

For more information, read more at the Windows Vista Get Ready site.