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.
http://www.microsoft.com/windows/products/windowsvista/buyorupgrade/capable.mspx
Ultimate has the most features. Its the most expensive but its what I would get.