iMacs do not appear to include video cards as part of the configuration:
20-inch: 2.66GHz NVIDIA GeForce 9400M graphics
24-inch: 2.66GHz NVIDIA GeForce 9400M graphics
24-inch: 2.93GHz NVIDIA GeForce GT 120 with 256MB memory
24-inch: 3.06GHz NVIDIA GeForce GT 130 with 512MB me
The NVidea SITE:
http://www.nvidia.com/object/geforce_family.html
To optimize:
http://www.nvidia.com/object/balancedpc.html
CPUs compute. GPUs let you experience. Both are important, but most computers today are shipped with an underpowered Graphics Processing Unit. Whether you’re editing photos, watching videos, playing a game, or just using the latest operating systems, an NVIDIA® GeForce® GPU will help your desktop or notebook PC run smoothly with jaw-dropping visuals.
http://www.trustedreviews.com/laptops/review/2008/10/21/nVidia-GeForce-9400M-Hands-On/p1
Classes of video cards:
http://www.notebookcheck.net/NVIDIA-GeForce-GTX-280M-SLI.17191.0.html
