Microsoft announces Vista minimum requirements
Hardware requirements for a Vista Capable PC are a modern processor with a speed of at least 800MHz, 512MB of system memory and a graphics processor that is DirectX 9 capable.
A Windows Vista Premium Ready PC has a good deal more requirements. It must have at least a 1GHz 32-bit (x86) or 64-bit (x64) processor, 1GB of system memory, 128MB of graphics memory, 40GB of hard drive capacity with 15GB free space, a DVD-ROM drive, audio output capabilities and Internet access capability.
It also needs a graphics processor that runs Windows Aero, which Microsoft defines as a DirectX 9 class graphics processor that supports a Windows Display Driver Model Driver, Pixel Shader 2.0 in hardware and 32 bits per pixel.