FIRSTLY DEPENDS ON WHICH VERSION
HERES A LINK TO THE PAGE ABOUT DIFFERENT VERSIONS
http://www.microsoft.com/windowsvista/getready/editions/default.mspx
A Windows Vista Capable PC includes at least:
* A modern processor (at least 800MHz1).
* 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.
* 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 Drive
* Audio output capability.
* Internet access capability.
YOU CAN DOWNLOAD THE UPDATE ADVISOR WHICH WILL TELL YOU IF YOUR COMPUTER IS COMPATIBLE
http://www.microsoft.com/windowsvista/getready/upgradeadvisor/default.mspx