The stock graphics card in the new Thunderbolt-y iMacs is AMD's Radeon HD 6970M, in 1GB or 2GB flavours. How powerful is it?
It was benchmarked a few months ago. The bottom line? At the time, it was pretty fast (for Modern Warfare 2, at 1920x1080 with 4x AA and the highest detail settings it averaged 77.4fps), but not as fast as the fastest mobile chip, Nvidia's GeForce GTX 480M series. [Tom's Hardware, Notebook Check]