Microsoft is finally firming up the requirements, and I guess you should soon be able to see PCs with stickers that say a PC is Vista Capable or Vista Premium Ready. If you already have a PC, skip to the end....
Vista Capable means a PC runs Vista but without the 3D user interface, Aero Glass. Microsoft says that includes at least:
A modern processor (at least 800MHz). 512 MB of system memory. A graphics processor that is DirectX 9 capable.
A Windows Vista Premium Ready PC includes at least:
1 GHz 32-bit (x86) or 64-bit (x64) processor. 1 GB of system memory. A graphics processor that runs Windows Aero. 128 MB of graphics memory. 40 GB of hard drive capacity with 15 GB free space. DVD-ROM Drive. Audio output capability. Internet access capability.
See this page for more info on the graphics requirements.
The minimum required spec is here.
If you already have a PC, go to Microsoft's upgrade page and download the Windows Vista Upgrade Advisor beta. (Note: it's English only, and requires Windows XP.) There's a FAQ about it.
