You can find out which graphics card you have by typing in "dxdiag" in the search box on the start menu. This will open up the DirectX Diagnostics Tool.

Once the DirectX tool has opened, select the "Display" tab. In the "Device" section, labelled "Name:" you should see the graphics card's identity.

If the card has "ATI" in its name, then you can download the latest drivers from here. Select the version of Vista (32-bit or 64-bit), the type of card and then the card itself as identified in the DirectX tool.

If the card has "Nvidia" in its name, then you can download the dirvers from here.

If in doubt over the number of bits your OS has, you can check by right clicking on "Computer" on the desktop and selecting properties. Under the "System" section labelled "System type:" you should find the number.

Good luck