Windows Vista has modest minimum system requirements that everyone with a system from within the last 5 years should have no problem meeting. Simply adding more system memory (RAM) is the most anyone with a less than 5 year old should have to do to meet them. The most noticeable change with Windows Vista will be the new Aero interface and for that you will need an Aero-capable DirectX 9-class graphics card. This is what the majority of older systems will need upgrades for. Since this is one of the main reasons to get Windows Vista just about everyone should avoid the Vista Home Basic and Vista Standard Editions since neither supports the new Aero interface.

Vista Requirements - Indepth