I ran Xbench on my new base-model iMac, and the OpenGL score is half that of my old Mac Mini 😕
Perhaps the 2400XT drivers haven't been fettled yet, or there's a problem with Xbench itself, but I was expecting the performance to be better than the old Intel 950GMA rubbish. Luckily I've no interest in gaming on this platform but I have other apps that use OpenGL, e.g. Starry Night, so this is rather disappointing.
I haven't checked the X1600 OpenGL scores at the Xbench site, but I've no doubt that they should outstrip the Intel onboard graphics. So the 2400XT's looking rather weak....