Because you didn't post your gardware, you find the hardware requirements for Vista also there:
According to Microsoft, computers which can run Windows Vista are classified as Vista Capable and Vista Premium Ready.[32] A Vista Capable or equivalent PC needs to have at least 800 MHz processor, 512 MB RAM and a DirectX 9 class graphics card, and will not be capable of supporting the high end Vista graphics, including the Aero user interface. A Vista Premium Ready PC will take advantage of Vista's "high-end" features but will need at least a 1.0 GHz processor, 1 GB main memory, and an Aero-compatible graphics card with at least 128 MB graphics memory and supporting the new WindowsDisplay Driver Model. (...)
I think, if you are using the PC for playing games, you at the moment will with an old computer have more fun with Win XP then with Windows Vista, because it leaves some more hardware ressources for the game itself