Here is the unofficial system requirements.
# CPU: x86-compatible 32-bit or x64-compatible 64-bit microprocessor(s) (Dual-core systems will be supported on all editions except Starter, and multiple physical processors are supported on Business, Enterprise and Ultimate editions)
# Motherboard: ACPI-compatible firmware is required.
# Memory: At least 512 megabytes of physical memory. Microsoft is also encouraging the use of ECC memory to improve stability.[1]
# Graphics Card: A DirectX 9?compatible GPU that is capable of supporting Windows Display Driver Model (WDDM) (only needed for aero glass) and has 64 megabytes of VRAM at minimum.
# Hard Drive space: At least 3 gigabytes for installation files, possibly more, depending on the edition of Windows Vista.
# Display: Copy-restricted high-definition digital content, such as next-generation HD DVD movies, can be displayed at a reduced resolution of 480p (DVD quality) or 540p if the copyright owner desires, unless viewed on a High-Bandwidth Digital Content Protection (HDCP)-compatible monitor. Very few existing displays and retail video cards are compliant with this standard.
More info about Vista over here -
http://en.wikipedia.org/wiki/Windows_Vista#System