A Windows Vista Capable PC needs at least:
And here are the specs for a Windows Vista Premium Ready PC, which can run Windows Aero:
A modern processor, at least 800MHz 512MB memory A DirectX 9 capable graphics card
More details over at Microsoft.
1GHz 32-bit or 64-bit processor 1GB memory A graphics card that runs Windows Aero (more details over here.) 128MB graphics memory 40GB HDD with at least 15GB free space DVD-ROM Drive Audio output capability Internet access capability