I just read from the other thread how people were saying the new iMac (24") is worse than the old one with a 7300GT card based on this report from Macworld:
Its also true that some of the benchmarks for ATI chips in the 2600 family show little improvement from previous generations running older games but significant improvements running newer ones (I assume that take advantage of the more advanced features of the chip).
At lot of different things go into 3D performance. Until we get a couple of people to bench the latest GPU eaters in the new systems we won't know how they will perform. We do already know that the "2600 Pro" in the new iMacs is NOT identical to the 2600 Pro benched 9 months ago by the internet sites.
The core chip is the same (and is the same as the Mobility 2600XT) the difference is the memory (DDR3 in the iMac) and the clock speeds on the GPU and memory in the custom card ATI built for Apple. (and the custom ROM, Drivers, and system they are embedded in). We need real hands on benchmarks to know what to expect (but so far it looks closer to the 2600XT than the "2600 Pro" numbers).