Click on the Apple icon in the upper left hand corner of your menu bar and select
About This Mac from the menu.
In the dialog that appears click on the
More Info... button to bring up the detailed about dialog.
Click on the
System Report... button and you'll get a split-window dialog that has hardware components down the left side and details on the right.
Graphics/Displays on the left. Pick a graphics card from the right side and you'll see all the details the system provides about it.
You can also get to the System Report by running Applications -> Utilities -> System Information.