The video card in the new iMac is a piece of trash. If gaming doesn't float your boat, you have no worries, but if you think you're going to be able to play even 1-2 year old games at the native resolution of the iMac's LCD display, think again:
http://www.anandtech.com/video/showdoc.aspx?i=3023&p=8
The above chart shows the results of a Prey timedemo running at 1600x1200. Prey was released over a year ago and is based on the Doom 3 engine, which was released in 2004. Since the result of a timedemo is an average framerate, you want to see results of 50-60 fps so that you can be sure you're not dipping down below 30 fps very often (which would cause the game to stutter). As you can see, the Radeon HD 2400 XT is at the very bottom of the chart with 14 fps, and the 2600 Pro is second to last with 25 fps. Note that the 2600 Pro does not even run the game as fast as the nVidia 7600 GT that was available in the last-generation 24'' iMac.
So much for Apple's games initiative... what a load of crock.